Sure it's an amazing amount of good quality work you did those past 17 years !
Here I don't know how to make a test case. It's linked to the database settings at the schema creation time, and how they are translated into jdbc metadata by drivers. My feeling is upgrade process from 5.7 to 8 doesn't give the same result than creating the schema on a fresh install of mysql8.
What I saw is the mapping made by the driver changed over time about lowercase_table_names variable to the jdbc metadata properties. Now, with mysql8, there are also changes in the dictionary and joins ... and recommendation to upgrade the jdbc driver...
thanks for the option, I missed it in the code. I finally rolled back to mysql 5.7 recreate everything with lowercase_table_names = 1. Application seems to work again, but I don't know yet how to upgrade to mysql 8...