I tested Mecha 8 and Mecha 9 with F18A firmware:
v1.4 = works fine
v1.5 = works fine
v1.6 = black screen
v1.7 = corrupted graphics (photos below).
v1.8 = black screen
It does not matter if you are using Atarimax Ultimate SD cart or original Pixelboy cart. Same issue in either case.
Please note SGM was connected to console for these tests.
If SGM is removed games boot ok, however without SGM sound.
Pressing reset has no effect