So... You'd have to do work to properly blackhole traffic when wg0 goes down. However long it takes to reconnect, you still will automatically fall back down to ens0 while it's down unless you do something to stop that.
Packets are always delivered to wg interface as long as it is marked as 'UP' (or 'enabled', if 'UP' sounds to you like having anything to do with some kind of "connection") when your routing table directs them there.
When they hit the wg interface when the other endpoint is unreachable for whatever reason, they are either dropped, or queued, or get ICMP unreachable response generated for them, depending on situation. This is done internally by wireguard.
If you wanted to not go through wg0 (whether it can reach the other wireguard peer or not) you'd have to remove the routes first.