This type of thing comes under what I call Strange, Odd and Curious Behavior, and S/O/C/B, especially when coupled with the fact that doing anything to the Form corrects the problem suggests corruption, in one form or another.
In this case it's probably corruption of the Form, or possibly, corruption of the entire app!
The first and easiest thing to attempt, for corruption, is to create a new, blank file and import everything into it. Only takes a minute or two to create and check out, and if it doesn't work you haven't really lost anything! And, amazingly, it does frequently work!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007