For a bike with a single chainring, Nothing should make the chain fall off. Chain too long?

I have had this issue when I had A.) a loose driveside crank and B.) a bad BB so the spindle could slide.

As mentioned by others, bent derailleur hanger ... or even the chain not threaded through the derailleur properly.

There is no "cross-chaining" on a 1x ( a bike with a single chainring.) If the bike shop guy told you there was, he is ... mistaken, charitably. Avoid him at all costs.