The issue here is that the UDF runner tries to determine if the subquery results should be cached based on whether the UDF takes parameters and the subquery is correlated.
In this case the answer is no in both cases, so the query result can be cached at the request level, but the fix to MB-53372 is partial and only checks if the parameter list is nil, and not if it has zero length.
The net result is that a corner case, where the UDF scans lots of documents (people shouldn't really be doing this, but still), and it is executed over lots of documents (people shouldn't be doing this, but still), now will take lots of time.
The fix is to extend the test of the parameter list.

To summarize - change in behaviour from 7.1.1, shouldn't happen a lot, when it happens it will be very annoying, easy fix.

Build couchbase-server-7.2.0-1852 contains query commit a85b778 with commit message:
MB-53420 consider empty non nil parameter list for inline UDF caching

Build couchbase-server-7.1.2-3421 contains query commit a486b26 with commit message:
MB-53420 consider empty non nil parameter list for inline UDF caching

Build couchbase-server-8.0.0-1085 contains query commit a85b778 with commit message:
MB-53420 consider empty non nil parameter list for inline UDF caching

verifeid in 7.1.2-3421, the query now runs in ~4s

Build couchbase-server-7.2.0-5000 contains query commit a486b26 with commit message:
MB-53420 consider empty non nil parameter list for inline UDF caching