IBM is saying OPS/MVS got in the way of starting DB2. They referenced this APAR:
https://www.ibm.com/support/pages/apar/II04773
Release : 13.5
Component : OPS/MVS
Customer DBA group issued incorrect command and inadvertently stopped the DB2MSTR. Subsequent incorrect commands were issued to start it. START DB2MSTR should have been issued instead of -START DB2MSTR
If support is required to further evaluate, provide the following documentation:
1. Archived OPSLOG
2. A copy of any involved rules