See attached.
Okay, I see this most clearly in Times New Roman, around 9 or 10 points. At 14-15 points, the issue almost is invisible.
It also happens with “S” (majuscule ‘s’).
It does not seem to appear in the compiled text with the “M” (PDF). (Nonsense text for this test)
Note, the “m” appears normal. No extra spaces. However, the “S” is iffy; sometimes it has extra space, sometimes not.