All right guys.
This is what I did last week. I wanted to wait a few days to test it out and then I updated to iOS 13.5.1. I also used bluetooth so I wanted to test whether that had to do anything with it or not.
Anyway, so far STEREO is working fine.
This is what I did:
1) After talking to Apple CS last week (using TextFree app, since I live in Argentina) they suggested me to first do an "Erase all contents and settings" and if that didn't work try Factory reset. No way I was going to perform a factory reset since I have my bank token app set up and if I'd done a Factory reset it wouldn't had worked anymore and I would have to go to an ATM to set it up again and here we are lockdowned due to the COVID-19. (sorry for the story)
2) I performed an Erase all contents and settings and tried the video thing. Still not working.
3) Turned off and on the phone. Still not working.
4) I said to myself f*** it and started setting up my phone. I, for one, remove all visual effects and **** like that. Since enabling "Reduce transparency" was causing a weird thingy with the grey colour I did a soft reset which solved they colour I said to myself... let's try the video thing again. Enabled Stereo and started to record myself and it worked fine... and until today is still working fine.
So long story short:
Try the Erase all contents and settings. Turn the phone off and on. Do a soft reset and then try the Stereo thing. Let me know how it works.
REMINDER: I don't know who put that the problem was solved cause disabling Stereo is just a workaround NOT a solution to record in stereo.