Hi,
Sounds interesting and yet strange. Has NetApp support reported any BUG ? Have they managed to reproduce this issue?
I am sure if you had to downgrade Ontap from 9.1P20 to P18, there must be something which prevented it.
In your DNS: Was there an DNS HOST(A) record resolving to Data LIF IP ?
Logs:
Any findings from the 'packet traces' ?
SECD logs?
Thanks!