Is it boat specific, or does it apply to all boats, that the
transmission should be put into reverse when the motor (inboard diesel)
is off during sailing?

Even when the boat manual says to do so, I frequently find the tranny
stuck in reverse when attempting to return to neutral before starting
the motor.

So, let the prop spin freely while under sail or lock it in place with
reverse?

And why is the tranny getting stuck in reverse?

Thanks,

Marty