As attached is a part of the computation (same as was in the previous picture). I don't know it if has any effect to the visualization of the division line, but the formula is inside a collapsible area.
Display problems in Prime were reported here quite some time, mainly concerning division lines.
Usually the cause is that Prime isn't scaling correctly and the effect is differently depending on the zoom factor you set in Prime, the screen resolution and the Windows scale. So chances are that when you upload your Prime sheet, others don't experience any problems.
The suggested solution used to be to try slightly different zoom factors in Prime.
In your case its possible that a workaround might also consist in using variable names without an underline character in the subscript - just give it a try.
No, and I also cannot reproduce the issue with the P7 file Stuart had posted. It may have to do with the fact the I am using a 4K display with a quite large windows scale - not sure, though.
Its not a problem with the Prime file itself but rather the program not scaling or rendering correctly in some cases of combination of zoom setting, scale factor and resolution. I can't be sure about but I don't think thats it a newly introduced bug which only affects Prime 7. Would be interesting if someone using an older Prime version can confirm that bug, too. So far (in this thread) the bug showed up for two people using P7.