In the https://wiki.mikrotik.com/wiki/Manual:Interface/PWR_Line#Supported_Hardware pointed that supported hardware serial number ends at /9xx and partially /8xx.
In the PL6400 and PL7400 brochure said that supported devices are /9xx, partially /8xx, and also /0xx.
Currently I found in stock HAP Mini with s/n ends at /1xx. Is it newer version wich works with PL6400 too?
/8xx is 2018
/9xx is 2019
/0xx is 2020
/1xx is 2021
Yours seems to be new. It should be compatible.