After buying three power banks (none worked) I decided to buy a USB-C tester from Aliexpress (search for 32816538942).
The tester reports that my BMAX Y13 charges at 15 volts, not 12 volts like I thought it would. If I connect any of my PD power banks the BMAX negotiates 9 volts and it doesn’t charge.
And yes, it is definitely a BMAX problem. I have one charger that doesn’t work if I connect it directly (tester shows 9 volts), but which does work if I put a HDMI dock between the charger and the BMAX. With the HDMI dock in between, the BMAX will charge at 15 volts.
@Octavio : I tried your power bank from Aliexpress (search for 4000191871773) but it also connects at 9 volts. 🙁