The user recently upgraded their instance and now their Service Level Agreements (SLAs and/or OLAs) are no longer attaching. These SLAs are attaching correctly in instances that have not yet been upgraded.

The user having upgraded is not the root of the issue. Rather, the issue is that both the Start conditions and the Stop conditions defined in the user's SLA Definitions are true simultaneously, so the expected task_sla record(s) did not attach.

A task_sla record will not attach to a parent record (Incident, RITM, etc) if both the Start conditions and Stop conditions are true at the same time.
To prevent this behavior from happening, the user can modify their Start and Stop conditions so that they are distinct, making it very unlikely for them to both be true simultaneously.