You cannot conduct meaningful tests over such short ranges.
There’s a fairly high chance that in the weak case, what you are actually seeing is the receiver being overloaded and detecting the signal on a different channel than it is actually being transmitted on. Typically it will also be received more strongly on the correct channel at the same time, but allegedly there are situations where it would not be.
If you want to do something meaningful, put the source hundreds of meters to a few kilometers away.
Of course another possibility could be a problem with cabling/adapters, such that the “better” antenna is not actually getting connected to the receiver.