I would guess that that the problem is with the stylesheet here. Just a guess.

Try to change the stylesheet to the Default and see if the problem persists (Format>Stylesheets). Or, take a look at the Option Inspector (Format>Option Inspector) for the Cell Frame Options and adjust those.

Hope this might help.

Os version / platform please?

I don't work for Wolfram and cannot fix your problem. You could contact Wolfram support, but I will suggest your OS is at fault and should first be contacted without bothering the Wolfram team.

Font systems are extremely complex (space curves, graphics subsystem compatibility, correct information in font files, correct "font substitutions" (correct means not substituted at all, usually)). It is often the case the best systems get them right 99.9% of the time. For non-professional systems the "lack of professionalism" will be much more evident. There are multiple software failures and hardware failures as well which can cause "chopped" fonts.

You will love your system and live with it or fix it yourself or lean toward looking at Mathematica as an appliance (run it on it's own pc that is professionally supported by big tech).