After some private messages and some more testing the issue seems to come from radius accounting not being correctly acknowledged by NPS. Disabling radius accounting by manually modifying the files /run/accel-pppd/sstp.conf and /run/accel-pppd/l2tp.conf files with a serverline reading:
@klase thank you so much for this. I was struggling and struggling with why my VyOS 1.1.8 config wasn’t working with 1.3.1 and I thought I had it figured out when I saw references to the need for proxy-arp since the update to accel-ppp from pppd, but that didn’t do it. Your feature request clearly made it in, since ‘set vpn l2tp remote-access authentication radius server disable-accounting’ now works and fixes this problem I was having. It was hard to track down because it appeared to not be an authentication problem, since I was authenticated successfully. Thanks again for doing the work to figure this out.