@digixx, i believe the core does that where it chooses the strong Wifi signal.
That’s what i notice using a hotspot within proximity and a home router sitting in the living room.
It’s interesting why it decides to go for the 1st floor AP instead! 
You can also try to erase the wifi credentials. (Place in listening mode, hold on the MODE button till it flashes blue rapidly)
Enter the basement credentials first followed by 1st floor. It shouldn’t be the case where the sequence matter but let’s try it if you want 