I’d say that at least I would expect them to behave the same (that’s the bug in my view).
Btw, in Seq mode (recorded sequence) it behaves like in Arp mode.
What the “correct” way is, is ultimately up to the Polyend team. For me, being able to hold notes that will continue playing after a double tap on Shift gives us more creative options. But now we’re moving into the territory of the wish I linked to in the opening.