PB6 is assigned as UART1 TX in RUI3. That means it assigns the pin to the UART drivers and interrupt on this pin already and it might not be possible to use it as generic IO.
I tested successfully PB5, PB12, PB2, PA15, PA9 and PA10 with interrupts. These pins are assigned as GPIO’s in RUI3.
Yes, this PIN works perfectly as interrupt. I only asked to understand the behavior. We use PB6, PB8, PB9 and PB10 to interrupt when FALLING. When our system start, only PB6 fire interrupt without nothing connected on PIN (all this thin is pulled-down with resistor on our pcb). Maybe when RUI is setting this PIN as TX, it put HIGH-LOW in it before our system start. Probably it is firing the interrupt.