Sans & Mono, probably hinting: "б" bowl taller than "o" at 24, 28 ppem
The bowl of "б" is correctly slightly smaller than "o" in the outlines yet is rendered taller at some sizes.
https://github.com/IBM/plex/blob/04c26256b9fb5cf19dfc33623d71588fb125cf60/IBM-Plex-Sans/fonts/complete/ttf/IBMPlexSans-Regular.ttf in Chrome 95 on Win10:

https://github.com/IBM/plex/blob/04c26256b9fb5cf19dfc33623d71588fb125cf60/IBM-Plex-Mono/fonts/complete/ttf/IBMPlexMono-Regular.ttf:

@ortoslon I see that. We will look into this.
@BoldMonday Hinting issue I presume?
Seems fixed in Sans https://github.com/IBM/plex/blob/003b8b0a6f56acd78e8fa78ddf471f71412e18b4/IBM-Plex-Sans/fonts/complete/ttf/IBMPlexSans-Regular.ttf. Mono wasn't updated since then.