If I recall, the A12 line, which internally is handled correctly because it plays built-in Millipede (w/Sara chip RAM), is not working correctly when you solder a cart connector to the circuit board numbered traces.
Games with RAM, advanced bank switch game carts, and The Supercharger fail because external A12 doesn't change.
I tried my plain 32K bank switched Berzerk hack, and that runs on the Flashback 2.
So any non-RAM 8K, 16K, 32K should play.
I seem to recall some Undocumented Opcodes cause some game programs to fail, but those would be mostly used in homebrew era games.
The Flashback 2 board needs probed to see if the A12 line can be picked up somewhere else.
Like when playing the built-in Millipede, is an external trace changing the way A12 should?
The Flashback 2+ has a ROM board with pins that hold the built-in games. Some of those pins have to connect to the "Atari on a Chip".
The early Flachback 2 had the ROMs under a black glob.