I've also been having this problem. I'm playing as the Borg and have started seeing both mine and the enemy fleet power dropping dramatically. The ships are getting a -110 percent damage modifier and a -85 percent fire rate modifier added. It goes away a while after the battle, though sometimes it takes longer than others. I have to attack a station with overwhelming power to defeat it before the modifier kicks in.

Originally posted by Orionpax:

I've noticed recently that when fleets engage there numbers drop substantially, it wasn't an issue as both sides suffered so i thought little of it however, i began to notice that star bases aren't affected leading to losses that make no since ie. 35k fleet losing to 16k station, its really starting to get frustrating when i lose a tech superior fleet to a weak enemy. Wondering if anyone else had noticed this

I believe the issue has something to do with the STNH Combat Speed Modifier that you can select from the Q. When I turned it off the issue went away and started again once I turned it back on.

All of my resources have high stock piles. I'm using 11.3 with just the STNH mod. (I also have Challenge Mode, Tech Slow Down Mode, Cannon Wars Mode, Cannon Mode, AI UFP Formation Mode, Lower Ship Speeds, Realistic Map Expansion (Beta), and More Wars Mode running from the Q. But only the combat speed mode changed anything.)
The antiborg combat traits isn't the issue since it's happening to all AI fleets as well as my own, always at the same time and same effect.