Comparing mqdb_dll.sql with uc_dll.sql, it appears that the indexes MQ(1|2)CPx_SchTime are still present for MQDB.
In an earlier version of v21.0, these indexes were deprecated but they were not dropped if the user was using the alternative schema (SQL Server only)
The indexes INDEX NK_MQ1CP00_SchTime and INDEX NK_MQ2CP00_SchTime were deprecated but not deleted in all places.
Solution:
Update to a fix version listed below or a newer version if available.
Fix version:
Component(s): Automation Engine
Automation.Engine 21.0.14 - Available
Automation.Engine 24.4.0 - Available
The reference for this fix is DE127650