Re-read Alex's second post in this thread. There's no bug here, just mechanics that aren't working the way you expect them to, and a display value that's mathematically 'correct' but - as you note - somewhat misleading.
Sensor Profile has never been a meaningful number on its own; it's Sensor Profile + other fleet's Sensor Strength that actually matters.
It doesn't matter if it never was.
Sustained burn doesn't say anything about affecting the enemy sensor strength nor does going dark. Yet they both do.
The problem here is that any random fleet goes to 1/4th of its normal circle using going dark. Phase fleets however are only put to 50%. This leads to normal fleets getting way too close to a phase fleet in effectiveness based solely on how the formula works.
We can drive this further by doing some math - if someone wants to. I made this post after trying out a couple of modulations and see the gameplay effect and phase fleet has an advantage, but it is so miniscule that you might argue it isn't buggy, but it certainly isn't nearly as functional as it should be either.
Are you taking transponder +1000 into consideration? It is otherwise not possible to cut 75% of detection range by going dark.
Yes you are right I just did 1/2 * 1/2 for 1/4 to express the problem of phase fleets having a null value and not profiting of going dark as much, but it doesn't turn out to be 1/4 but still 1/2 instead. They both get 50%, but the point was, that the numerical value is much smaller for the phantom fleet since it has 0 sensor profile and thus nothing to reduce (if we look away from the bugs currently going on like nebula, asteroid feels and so on diminishing the negative value)
So if we take two fleets that have 500 sensor strength and profile and being non phase, then they will see each other at 1000. If one goes dark it is 500 for a total deduction of 500
If we make the second fleet phase and big enough to go to 0 sensor profile, then the first fleet will still see it at 500 or going dark 250. For a total deduction of 250.
Now you go from a very small fleet to a very big phase fleet for a net reduction of 250 detect range between those two cases.
The tooltip for Sustained Burn e.g. states:
Also increases the range at which the fleet can be detected by 100%.
I feel like that's a pretty good indicator that it applies to the combined detection range, not just your portion of it. Perhaps it could be made even more explicit somehow.
So effectively the problem is that going dark puts a normal fleet at 1/4th of its normal circle, while a phase fleet is only put to half. In gameplay that means that I can achieve the desired effect (being hard to spot) almost as good as a full phase fleet with any non-ox fleet.
Not sure why it'd be 1/4th? But as far as the final point, yeah, well... reducing the range at which you're detected by 300+ - how much this matters really depends on what the sensor strength of the specific enemy fleet is. It could matter a lot, or a little. Though yeah in the case where you'd probably want it to matter - vs a strong-sensor fleet - it'd matter less.
That has nothing to do with phase fleet bonus being weak, but going dark and sustained burn both affecting sensor strength of the enemy and your own sensor profile having so little impact on the circle to begin with.
Like I said, I've already got a note to look at making the phase fleet bonus multiplicative, like the abilities. However, while this is a good discussion to have - it's really not one for this subforum. I'd really appreciate it if you could be more mindful of the rules here; it's important to keep the bug reports forum free of anything that's not very clearly about bugs, not even "a bug and some related discussion". Going to merge this thread with the other one since it's, well, the same discussion.
It also contained legitimate bugs though. Like the negative score of the phase fleet being cut in half by all multiplicative effects. Not a big issue, although maybe noticeable if you go to a 30 phase fleet.
As far as the tooltip is concerned I understand where you are coming from, but that tooltip really is worked off when you think of the sensor profile being doubled already you have no reason to assume, that the sensor strength of the enemy is also doubled.