Minolta XG-M strange flash quirk

Fresh batteries in a Minolta XG-M. Camera operates correctly on all shutter speed and aperture combinations, with no speedlight mounted.

When a Canon 155A speedlight is mounted, the camera and flash function normally.

When a Vivitar 2600D is mounted, the camera shutter opens when the release button is pressed, but it does not close, and the flash does not fire. When the flash is wiggled and removed from the hot shoe, the shutter closes and the flash fires. The flash does not seem to fit perfectly in the hot shoe. The flash functions normally off camera and on a Canon AE-1 Program.

Strange! I thought all simple speedlights like this would work on all 1980s vintage semi-automatic SLRs.

Does this make sense? Has anyone experienced this? Could it be possible the 2600D is not compatible with the XG-M?

Solved. Turns out the 2600D I have is a "C/N/R" or Canon/Nikon/Ricoh variant. The box indicates this, but the flash does not. I dug up a Sunpak 144D and this flash functions correctly on the XG-M body. It's been a long while since I tried to exercise this camera body and these flashes.