Same bill monthly, different approvals by insurance–how to argue?

Trying to simplify this but if I've gone to far, ask away…
I get a monthly shot associated with cancer treatment. The provider bills my insurance (Healthfirst Leaf in NY) roughly $1k. My EOB amount has been about $100 (before I hit my deductible). Note, in the past two years, the insurance approved amount has rarely remained exactly the same (as you might expect it to), but it's at least been in the same ballpark and mostly covered.
This was the case for January & February of this year, then the next four months they said the entire $1k goes against my deductible and they don't cover any of it. My doctor's office reviewed all the coding–nothing has changed. My initial appeal was denied–I even had a cs rep claim Jan/Feb were wrong (but haven't revised those EOB's). The EOB for July reverted to roughly the previous amount, but in the meantime, I owe $4k to my provider (currently on hold while I try to get this figured out).
Can anyone advise on next steps? I have a doctor's appointment this week so I'll be asking if there's anyone on their end who can help, but this is ridiculous and infuriating. TIA!

(reposted for flair)

submitted by /u/AmateurEnthusiast3K
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