I have the aqara’s linked directly to homey, I do not own the aqara bridge.
The flow I provided should not trigger when the connection fails, because it ignores any case when a humidity value is 0. So if the bathroom reports 0, the flow stops in the and. If the bathroom humidity changes when the other room is 0, the flow stops in the and.
The same should apply if you copy the humidity to a variable - just don’t do that when it is 0, humidity is never 0. It will only do it’s work when both values are correct.
There is a small problem in the case that the other room would come back from 0% humidity to a value that is more than 8% below the bathroom’s humidity. Because as long as the bathroom humidity does’t change, nothing happens. In that (probably rare) case you should consider a similar flow that checks if you need ventilation when the other room’s humidity changes.