There was some discussion of the scheme here:
http://forum.mikrotik.com/t/new-hap-ax-lite-lte/168377/1
…which implies it a diff upgrade scheme, but IDK.
I’d imagine trying the 7.19beta8 be easier to see if that works with .09 & be easier than trying unwind how the download works. The URL download scheme looks more complex than past modems, upgrading take a few a minutes. A lot of times the newer firmware requires something in RouterOS (and/or RouterBOOT), so that may be an alternative solution (if 7.19beta worked) & if it did NOT work, then there is bug.