I can verify this is an actual bug, I just reproduced it on GL-MT3000 4.7.0op24 using a Tailscale version updated by @admon 's script.
The ts.net does not get added to the field in question until the router is logged in to Tailscale. Or in my case, Headscale - I'm using tailscale login --login-server https://my.domain.here on the command line to log in, and connecting to my own Headscale server.
Prior mention: Script: Update Tailscale on (nearly) all devices - #212 by glinoob