my mi 9t is also l3 widevine.... my theory right now is the android security patch level hasnt approved by widevine yet. thats why its l3.... but its just my guest
edit: since the new xioami eu rom, have updated to the latest august security patch (aug 1, 2020 which applied on your note 7 and mi 9t and some others). widevine, i think havnt approved the security patch yet... so maybe wait for the next rom update. or maybe widevine itself will approve this security patch....just saying