I think this actually shouldn't raise an error for the property in that example. Due to the region-based isolation, NonSendable in your example is in disconnected region, so it is safe to pass it to another isolation via async call. I think there was a thread on false diagnostic for computed properties...
UPD. That one: https://forums.swift.org/t/why-does-sending-a-sendable-value-risk-causing-data-races