I don't think this is the same bug as the "known bug" for shrines [which seems to be about emblems], but if I've misunderstood the known bug, I apologize.
I go to Mab, I donate 1200 points of stuff, and then I use the favor to cut down my learning time, taking my favor down to 0.
When I mouse over the shrine, it says my favor is currently 1200 out 1500. If I click on "Check favor", it properly shows 0.
If I leave the area and come back, it still says 1200 out of 1500. If I reload, it sometimes clears it. Sometimes it doesn't. I've had it last between browser restarts.
One thing I did not think to check (sorry) was whether a different Mab shrine would show the same wrong balance with the mouse-over.
Again, apologies if it's the same bug.