Whenever I get a similar “odd” behavior in NAV, then my first thought is that it’s not “me” but NAV!
So I would check if Microsoft eventually released a fix for it (that’s the things they typically fix in the hotfixes).
There before spending too much time on it (especially if you say you didn’t do any programming to the page), then try to get the latest roll up. Unless of course your NAV is already very old!
The way to test is to create a test environment (unless you have a separate test setup already), and then only update the test environment to see if it fixes the problem. Typically that’s the first thing Microsoft ask, when you report an error.