First of all, yes I do understand this is just a game and everything within it is very simplified and approximated.
Issue A, sizes. Yes, I do understand it is impossible to sustain real scale, and all the distances and sizes are very conditional, but all of the celestial bodies have different sizes, peculiar hint for players to understand which objects are bigger than other. But that system have some defects.
Examples:
Achaman. Good white dwarf relative size example.
(https://i.imgur.com/1wPdBcz.png)
Arcadia. Acceptable white dwarf size example.
(https://i.imgur.com/tcsFnER.png)
Hybrasyl. White dwarfs can not be that big, it's compared to red and orange stars. Same problem with Zagan and randomly generated white dwarfs.
(https://i.imgur.com/LmPKneq.png)
And, yeah, randomly generated brown dwarfs shouldn't be that big either. Real stars of that type are no much bigger than gas giants. But this is minor issue compared to that white dwarfs on steroids.
Issue B. Damn orbital mechanic. No, I will not whine about elliptic orbits, inclinations and Kepler laws. No.
Black hole orbiting brown dwarf. That sequence of words makes the hair on the back of my head stirring.
(https://i.imgur.com/oaxESic.png)
And it is kind of common to have smaller stars in a center of a binary systems.
Also, nothing preventing a black hole from having less mass than a brown dwarf.
Spoiler
(https://memegenerator.net/img/instances/81081092/are-you-kidding-me.jpg)
That is theoretical minimum for a black hole. (https://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_limit)
And the brown dwarfs have theoretical maximum mass. (https://en.wikipedia.org/wiki/Brown_dwarf)
So, unless black hole is artificial, something prevent it to be less massive.
The upper mass for a neutron star is not the lowest mass for a black hole. And while we have not found any particularly small black holes this isn't terribly surprising. They are after all, both very small, and very dim. And while it seems unlikely that we would find one in binary with another star(for a variety of reasons) this is a video game after all and we can have cool things in it
You are right. I was not entirely correct. That should be written as: "That is theoretical minimum for a natural black hole as a result of stellar evolution."
Boring words
Smaller black holes can be:
-Initial black holes left from a Big Bang, they could be just like any small. Theoretical entities. If they ever exist, they are probably get some additional mass from that time, which makes them potentially heaviest objects in a Universe. Some scientists consider them as initial seeds for super-massive black holes in a centers of the galaxies. If miracle happen, and some do not get extra mass - they are either microscopical and evaporate for that time, or out of common galaxy lifecycle, as this is the only way not to consume gas and grow. Being out of common cycle means really low chances for stable gravity partnership, i.e. no binary systems or planets.
-Degraded A-1 black holes. Theoretically, black holes losing they mass depending on size, Hoking radiation thing. Absolutely theoretically. Not everyone agreed here. So, neutron star, collapsed into a black hole because of additional captured mass, can loose some weight, if there are no new matter to eat. Eventually, those objects would be smaller than Tolman–Oppenheimer–Volkoff limit, still remaining as black hole. But that is incredibly slow process, even theoretically they are not existed yet from a start of the Big Bang.
-Quantum singularities. Nominally they are black holes, and they are exist almost for sure. Problem is, they are so unstable, that disappear sooner, than able to consume anything and grow to macro-sizes.
-Artificial black holes. There are no mass limitation, if you force matter to collapse artificially. Only barrier - tech required. But those objects are made for purpose and will not spontaneously spawn in a random not colonized system. And yes, creation purpose should exist.
-Yet unknown natural physical process. I mean, SS have a hyperspase and gates - same "yet unknown" category. If SS contains that black holes, isn't it should be described somewhere? That is almost a fiction rule. Aragorn was not wearing pants, because Tolkien never mention he was.
I agree, that searching a low-mass black holes is extremely hard task, if they exist. And there are no surprise we haven't discover them yet. On the other hand, we didn't discover a Russell's teapot either.
Spoiler
(https://i2.wp.com/cdn.howtofind.com/wp-content/uploads/2019/07/03192311/Russells-Teapot-How-Do-We-Think-About-Gods-Existence.jpg?fit=1920%2C1178&ssl=1)
you did just change the goalposts from a brown dwarf (in the original post, as well as quotes) to a neutron star. These objects are, in no exaggeration whatsoever, on opposite ends of the mass spectrum.
He didn't, actually. Link I posted related to a neutron star upper limit.
1. As was mentioned, initial black holes have little chances to gain stable orbit in such systems. Requires to be captured, and that is tricky gravity thing, which became even trickier due event horizon existed in body system.
2. It's description says it was a massive star.
Spoiler
(https://i.imgur.com/msP89qf.jpg)