I got the same issue with my Redmi Note 12 pro 5G.
In WhatsApp Video calls the other side doesn't hear me well, hear me way too quiet, mumbling or doesn't hear me at all.
The main mic seems to work very well outside of video calls (when the speaker is not outputting sound). But as soon as you use the speaker in calls or video calls, the mic no longer works properly and quality drops instantly.