Having strange issue with download speed, only download speed. I suspect something to do with the wireless?

I have 2 pack M5 router at home, say "Router A" and "Router B", they are connected using cable backhaul, and internet service with 80Mbps down / 90Mbps up.

• When "Router A" as a main router in first floor and my iPhone or MacBook connected to it (5Ghz), I only get 40Mbps down, but upload speed maxed out to 90Mbps. If I connected to "Router B" in second floor (5Ghz) I can get full speed both download/upload.
• When "Router B" as a main router in first floor, devices in first floor can get full speed. While devices on the second floor which connected to "Router A" can only get 40Mbps down / 90Mbps up.

The issue started only a few days ago. I have reset both router, and yesterday also upgraded to latest firmware, but still no luck.

Any help would be greatly appreciated.

---EDIT

I tried disable 5Ghz, when connected to "Router A" via 2.4Ghz I can get full 80Mbps download speed, but only get 40Mbps upload speed. Confused..