This bug is now happening on every past save I have had and new game I start so have done some of my own testing and on 3 saves here are the results
4 hours played 7 saves no issues, then built my boat and the next save corrupted with this issue.
the next 2 test were from the same original save about 5 mins into a new character (this made sure that I had the same starting point for multiple tests)
1 hour played total of 6 quit, save and continues with no issues, then I built my boat and the next save corrupted to have this issue
1 hour played 2 quit, save and continue no issues, then I built my boat and the next save corrupted with this issue
Only time I am facing this corruption is after my Ship is built (large ship)
I then did a further test as on the original save above as I had also built the additional ship parts on the front and back and the helm then had issues placing the mast as it said out of range, and the placement grid was red not white.
All in all with the large ship built and after being able to place the front and back additional pieces and the wheel with no placement issues the save, quit and continue worked fine and I was able to continue playing, but as soon as I built and tried to place a mast, I had the same placement issues with the boat being unstable and the mast placement grid always showing red, I was forced to place the mast in free view mode but the next save was corrupted.
If this is of any help it seems to be something to do with the mast placement issues where the whole ship seems to rock under the weight of the mast trying to be placed.
This is a game breaking bug for me as I am at the stage in the game where every play through this is happening and I am unwilling to spend anymore time in Ylands until this is fixed, to ensure I don't burn myself out on the frrst 1 hour of gameplay over and over before being faced with this bug every time I build a ship.