Right now if I try to use the Steam Import functionality I get this screen:
https://ibb.co/5Y2Xx47
which would suggest that everything is up-to-date and imported, there were no games found on my steam library that are missing on Completionator, etc.

However I can see that I have several titles that are not in my Completionator collection but are on my Steam, so at the very least the "Games Missing from Completionator" section should show them...

for example:
https://ibb.co/M2h5vRw
https://ibb.co/1vT42kM
https://store.steampowered.com/app/954150/Nowhere_Girl/

https://ibb.co/HFcR9ZH
https://ibb.co/1mJkyKT
https://store.steampowered.com/app/1096240/7_Lives/

and few others (Borderus, the Hentai Mosaic games, etc)
So those games are for some reason not detected by the import process, not sure why.

Hmm...I'm not sure on this one. I'm assuming you're still having the issue? I'll probably have to debug this one as the import seems to be working correctly for me and I verified those games weren't excluded (on this end) for some reason.

yeah, I've tried normal import and import with "include free to play games" selected - in both cases same result - "0 to import, 0 missing".
Few other examples:
https://store.steampowered.com/app/306290/762_Hard_Life/ - I have it on Steam, missing from my completionator collection page, missing in completionator database. I'm guessing this one may be treated as "same as 7.62 High Calibre" as it is essentially a fanmade enhanced version.
https://store.steampowered.com/app/925340/A_Winters_Daydream/ - same situation, got it on steam, missing on completionator
https://store.steampowered.com/app/904810/Ectolibrium/
https://store.steampowered.com/app/861940/Crashbots/ - have on steam, missing on my completionator collection, this one is present in completionator database

I think I may have figured this one out. I think it has to do with games that have "Profile Features Limited" on the store page. That doesn't explain 7.62 Hard Life, but your other examples appear to be in this state.