Hm,… there only seems to be the pattern of two days in a row without due reviews where I instead do a quick cram session. Then it seems to stop counting the days studied on the second day.
Other than that and that I did reset my whole progress on Jan 28th this year, I don’t think I use Bunpro differently than anyone else. I am not tech savvy, so I have nothing that messes with the service. Not using a VPN, my few extensions are ad blockers and focus on YT (like unhook) or are userscripts in tampermonkey for WaniKani.
I do use the app very rarely, but I always make sure it syncs with the website.
99.9 % of the time I use the Bunpro website on my PC for my studies.
So my best bet is the situation of 2 days no due reviews in a row not registering the second day of doing cram sessions as a “day studied”, because it now demands a new studied grammar point/actual review?
I don’t think the reset has an impact just because the heatmap now differs from my streak/days studied.
Edit:
Maybe we could test that: I don’t have reviews due for the next two days (did my reviews for today). So I’d expect the days studied counter will be 2 days lower when I did my cram session on the Aug 20th.
For documentation: This is how my stats look after I did my reviews for today.

Tomorrow it should say “streak 830” + “days studied 829”
And the day after “streak 831” + “days studied 829”?
I’ll post a screenshot of these stats for the next two days.
Edit 2:
As expected, today it says 830/829:

Depending on what it says tomorrow with the second day in a row without due reviews, I’ll either update this post again or post a new one tagging you again, Sean.