Excel changes font size when I change color / bold / etc.

I have an annoying problem with Excel 2010. My default font size is 10 points. If I select a PORTION of the text within a cell, and then change the properties of that selected text (change the font color, bold it, italicize it, etc.), Excel also changes that text to 8 point size. I have to then manually set it back to 10 points.

This only happens when I FIRST change the format of a portion of the text within a particular cell. After that, I can change the format of other portions of the text (within the same cell) and Excel will not alter the font size. I can also edit other cells, then come back and make further edits to the original cell, and again the font size will not be altered.

If I merely select a cell, and change the color or bolding of the entire cell (i.e. ALL the text in the cell), the font size also remains unchanged. The "automatic" change from 10-point (default) to 8-point ONLY occurs the FIRST time that I edit a PORTION of the text within a particular cell. But this is still very annoying, and I can find no explanation of why this is happening. Is there any way to prevent this?

Further detail: Cells contain lengthy text entries (in default 10-point black normal text). I am highlighting certain portions of that text by changing them to bold red text (still 10-point). When I make the very first format change within a cell (whether I click the bold icon, or the font color [red] icon), my selected text is also changed to 8-point size.

I have the same problem three years later, running Excel 365.

It happens for me when sheet zoom level is set to 80. My workaround is to set the zoom level to 100 while making these changes.

Not a satisfying answer but hope it helps someone!

Although this has bugged me for ages, I just discovered this zoom explanation for the odd behaviour today (Oct 2022).

But it's certainly not new - apparently it's existed since at least 2004 as you can see from this Mr Excel forum post:
(mrexcel.com) board/threads/cell-font-size-changes-when-bold-added.114543/post-566070

If the bug has lasted 18 years, I don't expect it's going to be fixed anytime soon.