Between the K-bikes and R-bikes I had, a few times the trans popped out of gear into false neutral. I assumed that I have not been careful enough with complete shifts, let it linger on edge of engagement.
Your situation is somewhat different, because you stayed in gear - but maybe similar enough?
The behavior of gearbox you describe seems normal to me. Preload when shifting seems to be the key with these clunkers while moving. When standing still holding the clutch, the dry clutch lets the gears spin down almost completely (unlike in a wet-clutch box) and they will not mate - some movement in box is required.
Thus, +1 on the advice to wait and see.