I couldn't quite reproduce this behavior on my LTSC 2019 v1809 system - I guess only some LTSC systems will get them and others (like mine) won't. can you try reproducing this for v1903 or v1909 to see if the preview updates for those versions get offered?
It's my main system, nothing tinkered with, just ran WU to see if there were any new updates and there were.