I got the same issue, man. I have tried clearing cache and stuff but the widevine on my phone is still L3
This is might be showing due to your account is login into another device which doesn't have wide vine L1 support . So logout from the other phone and log into one plus phone again then it will support.
I don't think it works that way. Also, it shows L3 in all widevine checker applications
report this here
AMA - OnePlus Support Dec Edition
by Tom B., 2020-11-30