The lookup read code seems ok. the error suggests that the table Ens.Util.LookupTable doesn't exist in SQL DB.
Are you sure that table you can successfully call from the portal is not local in IRIS?

Very good article @Dmitry Maslennikov 

The $LIST framework was created alongside classes in Caché as a replacement for the older $PIECE function. With $PIECE, you must predefine a delimiter that cannot occur in the data, while $LIST avoids this constraint entirely.

AI bot answer is not relevant to the question.

You say that code/data is in the same DB that is mirrored. 2025 and 2022 are using different version of journaling, but 2025 is capable of de-journaling journals from previous version (but not the vice versa).

1. When you upgraded the failover to 2025 (the one which is the new primary) did you had any issues with journal files? (might be that some files were dropped in the upgrade and not processed).
2. is that "missing rule" mapped to another DB that is not in mirror?