I agree it’s confusing, as one could “see” a property which wasn’t set. But what if javax.persistence.nonJtaDataSource was being passed at creation time of the PU - would it still be odd to be able to read it? I think it would be equally odd if the property would “disappear” after having set it.

I have no strong opinion on either way, both doing it or not doing it has consistency tradeoffs. I guess I’d allow to read the property as apparently there’s some use case for that, even though it’s questionable, as long as it’s not problematic to implement - and as long as there is no memory and performance cost in creating EntityManager.