Yes i see; i can confirm what you wrote here.
The "_" has been removed as it should be, so the result for web1_db1 is: web1db1. But if more databases are added, the database name remains to be web1db1, not web1db2 or web1db3 for example.
This is not how it should work and i think this is a serious bug which needs to be fixed!
The result is that only one database for a site can be created with ISPConfig 2.2.20.