Just chiming in that I've run into this issue as well. I had to manually reset my session target this morning. At least for me, the behavior has been inconsistent. Some days it resets as expected and other days it does not.
This is an ongoing issue that I assume is late on the list of things to sort out, It's been like this for many beta versions.
Also, if you you don't have allow negatives ticked, and you cut more text than you have written for the day and paste it, your word count will reflect the number of words in the pasted text. Cutting more than you've written without allowing for negatives resets the word count to zero and then the pasted text becomes your new word count. So tick that box whether you want to allow negatives or not.