: Has anyone experienced this and what can I do?

The answer to your last question is that it happens to everyone who leaves just one side of the hull or deck glassed if there is a change in humidity or some other change that affects the moisture level of the timber.

With some timbers, possibly with most timbers, it only takes a small reduction in the moisture level for the hull to curl in or an increase for it to curl out.

As has been explained, getting some moisture back in the timber will restore things to normal if you are patient.

It is good practice to get both sides glassed without a big delay between the sides to avoid that problem. The people who have delayed and not had a problem have either been lucky the humidity didn't change or perhaps they work in a climate controlled workshop.

The important thing is that your curl can be fixed. There is no need to force things enough to cause cracking.