I recently assaulted the enemy homeworld and am getting some weird, related issues. First, I destroyed their shipyards using beam ships, and while the shipyard signature disappeared, the ships kept firing and I kept getting events saying slipways were being destroyed. Finally I just stopped and moved on with the invasion. After I captured the planet, it is showing about -90 million works in the shipbuilding industry on the Summary tab. On the Shipyards tab, it lists two shipyards, both Naval type, with -14 and -28 Slipways. The Available Slipways shows the same negative numbers. The Mod rate is still positive, though. There are no assigned classes, and no current activity, and the Capacity is positive for both (11,200 and 8000, respectively).
This may potentially be related to the one the new member above just posted, too.
***
Update 8/28/21: I tried retooling, and it let me, so I tried ordering construction and it just made the number of slots available go further negative. . . forever. I now have infinite slipways! As cool/handy as that it, that's a bridge too far for me (not to mention the -90M pop I get for free), so I deleted the two bugged shipyards and everything appears to be back to normal now. Still may be a bug worth hunting down and fixing if it's easy, but it's not a game-breaker and you can fix it yourself, so low priority.