We've been fighting this through the lifetime of Ignition 8 - the backing library behind the majority of the code editors you see in the designer is getting a bit long in the tooth, and seems to not handle our bundled fonts properly. We're aware of the issue, and might slap some bandaids on in the short term, but the medium to long term fix is to replace the code editors with a newer one, which should also hopefully support nicer features like code folding 
If you can come up with a way to reliably reproduce something like that, we'd like to know about it I'm not aware of any known bugs that would lead to something like that.