Singleton SELECT inside cursor loop

AWS RDS Postgresql 12.11

It runs quickly, no matter what the fetched cursor values are, as long as 
the cursor returns less than 8 rows.  After that, the singleton SELECT's 
performance tanks.

We've got plenty of other procedures which have a similar "Singleton SELECT 
inside a cursor" design (many even query the same table that the problematic 
query does), but only this procedure's singleton SELECT tanks if the cursor 
returns more than 8 rows.

It only runs once a month, so we've only just noticed the problem.

Why would that be?  (We recently updated from 12.10 to 12.11, if that's 
relevant.)

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