I have similar bug with ASA to ASA (VTI).
Tunnel flaps after Phase 2.
The debug output below:
Jan 15 14:52:25 [IKEv1]Group = x.x.x.x, IP = x.x.x.x, PHASE 2 COMPLETED (msgid=8672abd6)
Jan 15 14:52:31 [IKEv1 DEBUG]Group = x.x.x.x, IP = x.x.x.x, sending delete/delete with reason message
ASA versions are 9.8(2) and 9.8(3)