I'm on 2.4.1 and yes the same issue. I filed a bug with my theme developer thinking it was an issue with the theme as everything shows in stock but they tried it on the default Luma theme and it does the same thing. Has anyone filed a bug for this?
Not sure if helps, but try.
Stores > Catalog > Inventory > Out-of-Stock Threshold
Make sure its 0, had the same issue with 2.4
I had the same problem on 2.4.1. First I tried to disable the whole MSI, using the instructions here: https://meetanshi.com/blog/disable-magento-msi/
Bad idea. Messed up my checkout.
I then installed the Disable Stock Reservation extension https://github.com/magesuite/disable-stock-reservation and so far it's working perfectly. I see there aren't any outstanding issues on the extension and the last update was on 9 December 2020.
Hopefully this issue will be resolved in the next Magento update.