After the most recent update an underwater monument and a nether fortress moved location in one of my world's. Before the update the nether fortress was at 176, ?, -112. But after the update it was at 384, ?, 272...
I don't remember the precise coordinates for the monument, but it's moved over 800 blocks away, at least.
Another workaround, that seems to work most of the time is:
Make sure you disembark in no more than 1 block deep of water and look directly down while exiting.
Doesn't always work, but most of the time
I can confirm that most of the time the reported work around where you disembark whilst looking down in only 1 blocks worth of water usually means the boat stays.
Having the same issue. Super frustrating having to keep creating boats if you don't want to do the above solutions...
Ps. I collected blaze rods from the previous fortress only to find that when I went to return to get more it'd moved...