Thank you for replying.
Seems that both classes exist in the core and both have css attributes in the theme.css (admin folder), but not sure why long ago I don't remember seeing it.
And, after checking the file, the font-size attr for the class '.pstaggerTag .pstaggerClosingCross' already has 0 as its value. So, something is overriding it with 1rem.
Update: It seems that the Element Flow module loads a file (admin.css) which overrides it, maybe a mistake by the devs. I already reported it.
Thank you for your help!