If you import from our addon, it will read your artifact item level correctly.
Not picking up the off-hand runeforge when your frost artifact is not currently equipped is a separate issue… one that as you said is an irritating side effect of how the off-hand weapon isn’t really there when it’s in your bags. We’ll probably just never fix that one since artifacts are going away soon.
I can look into why it’s off when reading from the armory… I know that it works for e.g. my retribution paladin with a single weapon. I’ll see if I can figure out what kind of data the armory is sending for two weapons. I admittedly didn’t put much effort into that during Legion because the armory could not read a lot of relic and crucible information at all, so it wasn’t of much use to anyone optimizing. Most people moved to using the addon exclusively to replace the armory.