I’m having this issue as well. Yesterday I opened a new text document within the Draft and wrote 1076 words. Writing History shows 0 words written in the Draft yesterday (and for all of December) and 10 words elsewhere, probably in a Reference document. I’m using Big Sur.
TL;DR: TO WORK AROUND THIS BUG, DISABLE “RESET SESSION COUNT”
Project->Show Project Targets->Options…->Session Target
Under “Reset session count:” choose “Never”
Long Version:
I started a new project about a month ago. Knowing the writing history is flaky, I have been meticulously recording my daily word count in a spreadsheet. Because I’m a slacker and sometimes forget to record my data, I set the “Reset session count” to Never. When I begin a writing session, I double-check my spreadsheet to be sure I recorded the last one, then manually reset the session counter. (I also have Allow negatives checked, and Count text written anywhere in the project unchecked, but I don’t think these have any bearing on the Writing History bug.)
Today I checked my counts against the Writing History and found they matched exactly. No words have been dropped from Writing History in a month of writing. Given the frequency of the bug in my other projects, I am pretty confident that the Writing History bug is caused by some kind of interaction with the automated session reset (which defaults to resetting at midnight each night).
If you feel like it, you might try experimenting with other values. I suspect that “On project close” would also be “safe”, but I don’t want to lose my session count when I close, because I might have forgotten to record it.
It might also be interesting to see whether manually resetting the session counter immediately after the session affects Writing History. (I always reset mine the next day. I never know when I might sneak in some late night drafting!)
Hope this info helps some folks out. Happy writing!
– Vince
On Thursday evening before I had 3163 words, in Writing History. The following morning it said -27 Thursday count! I had not deleted any and the total word count is correct in statistics. but not inWriting History. I haven’t changed any of the settings.
I saw there are some old posts about this but nothing for 2 years, so wonder if anyone else has experienced this. I am using the NaNoWriMo template but chose not to auto-update my words to NaNoWriMo as I have two projects running this month.
I’ve merged this with an existing discussion on the bug. There appears to be a workaround suggested above.
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Thanks, doesn’t really help get back the old stuff but will try… it’s been around a few years this bug I would have thought it might be addressed by now 
We’ve never been able to reproduce it, and it’s also a really tough one to reproduce since it requires variables like time of the day and actual activity, etc. I ran dozens of tests back in the day, messing with the system clock to simulate midnight with as many permutations of settings I could think to try, but it always successfully carried over the word count.
My Writing History on 3 occasions over the last week has been wrong. I wrote over 2.5k a few days ago and the next day my word count in Writing History for the prior day was “0”. This is getting frustrating as it’s throwing off my writing tallies that I’m trying to track. I’m going to have to revert to taking my word count at the end of the day and storing it externally.