It’s many years since I flew and taught on piston engined aircraft but the magneto check I was taught (with a combined rotary switch) was:
(From BOTH). L = drop no stop, BOTH. R = drop no stop, BOTH. OFF = drop and stop, BOTH.
If you don’t see a drop, or an RPM recovery at BOTH, the check is a failure.
I have known switches go “erratic” when the internal contacts were worn.
The engine was always shut down using the mixture control knob pulled to “cutoff”. That proves both systems. You were either doubly unlucky, or someone hadn’t been doing this.
As for the maintenance.....