This bug is related to the new Awk mage pet buff introduced several weeks ago that gives the caster an additional 700 total MP. Upon entering AoS and using the pet summon, MP will reset to 0. When the timer to round start hits ~7s remaining, it will reset again.
Step:
1. Enter AoS
2. Summon Pet (only tested with Gorr thus far)
3. Mana Bar decreases to 0
4. at 0:07 let until round start, MP resets to 0 again
5. At start of new round (not match), repeat from step 3