Also encountered this bug, nothing special about the situation just a normal head-on engagement vs random pathers. Medium size fleet on both sides. I hit deploy all and noticed something was off. I had memory at around 3k (as I had a memory soft warning from the game earlier - surprised me a bit, running no mods + the default value, so I doubled it) and increasing it to 4k like ProNoob wrote fixed the issue here too.
What I noticed in addition to the OP's description of the issue, is that launching a command shuttle to switch to a different ship, the command shuttle was stuck next to my flagship at 0 speed for quite a few seconds before moving normally (seen on the command overlay)
And if I panned the empty view up as far as it would go, I eventually arrived at a white screen. I'd assume out of bounds or something but the sound effects matched normal fleet action while I was in the command overlay. So I don't really have any clue what this is.
I'm running RC9 at normal UI scale, 1920x1080, no antialiasing. LazyLib and Autosave are the only mods enabled.