I have also noticed this same issue testing missions on my no-render dedicated server machine. I had previously thought it down to my server PC not being powerful enough as it’s a heavy mission, however, last time this happened, and as you say, the aircraft (all AI except myself flying) froze position, I could still cause damage and the flak was still firing on them. The rest of the mission was still functioning - artillery was firing, flak still firing, ground units still active. The only units which froze up were the aircraft. After flying around and waiting they occasionally and randomly switched positions and ‘updated’. One occasion it’s actually fixed itself and the battle continued without another freeze.
This mission uses exclusively WW2 units by the way.
I checked the status of the server during the freezes and it was intermittently ‘not responding’, but always came back and never actually fully crashed.
I’ll do some more testing and try and reproduce it. For me this is encouraging in a way as it may not mean my machine
isn’t up to it:)
Edit: also to note this was occurring before the 2.5.6 update as well as after.