The #5 position ship in my fleet changes the type of wood that it's made out of if I leave it in a port overnight (either staying at sea or leaving it on the beach seems fine, with no apparent issues). I first noticed this when one of my Snekkja downgraded itself from Oak to Ash, so I started experimenting a bit, and discovered that the bug seems to only affect the #5 ship slot in the fleet, no matter what ship is in said position.
Thinking that this could be an issue with my particular save(s) being screwy, I started up a brand new Story Campaign to test it out. As soon as the initial Woden Ric battle was over, I cheated myself a lot of gold, and teleported around between Lundenwic, Dorestad, and Ribe, buying up a total of 6 ships, then repaired them all and started waiting in the port at Ribe. Sure enough, my #5 ship (starting out as a Pine Skeid) downgraded itself to Ash, then upgraded to Oak after waiting another day or two. I wasn't familiar enough with this particular fleet to be 100% sure that none of the other ships changed as well, but I think they all remained the same.