I'm having this problem too on my Sasmsung Galaxy S20 Ultra. This is either some kind of malewre that has compromised our phones or a really crappy bug that has not been found. I spent 2 hours with Google Fi and after rebooting the phone into safe mode and watching it work there, then having SMS messages become delayed, for hours running normally. They suggested it was a 3rd party app. IDK, what it could be I'm spending hours chasing this issue and changing 2FA solutions because I can't trust this phone to recieve messages anymore.
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