I'm not so sure that checking just the left side will be correct for composite keys.
This query works under pgAdmin, but I haven't tested through Hibernate. I don't even know if I can give such parameters if they're not a composite primary key class. I'll get back to you on that, hopefully within 2 weeks. I have other things to do and it doesn't seem like a big priority.

Fixed in master and 5.0 branches.

Thanks for the tip about StringHelper. You were close. Hibernate should only check for a left parenthesis before the parameter to determine if it should be enclosed.

Just checked with 5.0.6. No change.
Debugging lead me to org.hibernate.internal.util.StringHelper where in public static String replace (line 175-179) there is a check if it's already in parenthesis, however the logic of the check seems wrong.

If it has a closing parenthesis on the right it will set encloseInParens to false, even without having the opening one.
I don't remember how the specific boolean logic law was called, but formulated it's something like this !a && !b == !(a || b). Essentially, I believe the problem could be solved by changing the condition to:

I tested the change and it works for my queries, but it might break something for the cases where you've done workarounds. Such old workarounds might have to be removed.

was fixed in 5.0.6. Please check to see if that fixes your issue.