FS GPON ONU stick in a UDM-Pro: serial written with set_serial_number does not stick

I want the ISP box out of the meter cupboard and the GPON line terminated straight on the router. It is a KPN-based line, fibre comes in on SC/APC, and the idea is to move the WAN from the Huawei ONT into the SFP+ cage of the UDM-Pro and keep exactly the VLAN 6 / PPPoE setup the ONT was running.

Kit on the bench:

• Ubiquiti UDM-Pro, WAN moved to the SFP+ port
• FS GPON ONU stick with MAC SFP, industrial version (1310 nm TX / 1490 nm RX, Class B+, DOM, simplex SC/APC), FS item 133619
• the ISP-supplied Huawei ONT, whose HWT-prefixed serial I read from its admin page at 192.168.18.1 with the Epuser/userEp login
• a temporary 192.168.1.0/24 management network in UniFi so the module's default 192.168.1.10 is reachable

SSH into the stick works, and cloning the serial appears to go through without complaint:

set_serial_number HWTXXXXXXXX
fw_printenv
# the serial I just wrote is nowhere in the output

On top of that, Port Manager shows the SFP+ cage as empty maybe half the time I look at it.

What I have tried so far:

• reseated the module and power-cycled the whole thing; the management address comes back, the serial does not
• kept the live fibre attached while it boots, since it clearly does not come all the way up on a dark port
• moved it to the other SFP+ cage, where detection is noticeably better, but the serial still does not stick

Is set_serial_number supposed to commit by itself, or does something else have to run before the value lands in the environment? And is the on-and-off detection in Port Manager normal for these sticks, or is that the cage telling me something?