It is a bit hard to see, and sadly i don't have a save from a few turns earlier, but as you can see the red line for fleet power makes a gaint zigzag. The upward line is me building titans non-stop, the downward line is me scrapping 2 frigate's, 3 destroyers and a cruiser. Those where the ships i had BEFORE the upward line... So the weird thing is that the drop is soo much greater then the fleet strength they represented before that... bit weird... Didn't lose any combat ships, lost/gained a number of assault ships but not that many in those turns....
Had you put some ships (including Titans) for refit during that zigzag? Putting ships to refit may cause downward spikes, depending on how much fleet strength they represent. Let me know if you recall doing that.
Good call.. yes i did... was able to load auto save from a few turns before.... Learned something Also... It happen to be when an AI declared on me.. guess they factor fleet strength into it?