yep just had the same thing happen to me on my computer browser not even on my phone, and just posted here too!!! I thank Brave broswers ran updates and wiped out all our BAT for the month before we were paid!!! They need to resolve this, this is BAD and has happened before!!!
I noticed the problem this morning, and then continued using the browser normally. After less than an hour of browsing, I checked brave://rewards/ again and the BAT was showing the proper amount.