Focus is already on the Control while it's LostFocus event is firing, and as the OP states, why would it work for two Controls but not the third one?
In testing, the posted code works, even if the Control is empty, without erroring out, so the code, itself, is not at fault.
Have you tried testing the code on yet another Memo Field Control? I'd do that, even if you have to temporarily create another Field and Control to do so, to see if it's only this Control that has the problem.
Although we usually think of Forms, or entire Databases, when we speak of corruption, individual Controls, can and do become corrupted. The test and the fix are one and the same; deleting the Control then re-creating it! If it was corrupt, you've verified the fact and solved the problem! If not, you've only lost a minute or two!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007