What other spheres are you using?

Power Gambit, Power Overwhelming, and Mana Leech

Are you referring to starting with 200 mana but a 300 cap, or you are capped at 200 mana? Starting at 200 mana but having a 300 mana cap may be intended, but I'll have to run it by design. If you are starting with a 200 mana cap, that's definitely not supposed to happen. I haven't been able to reproduce starting with a 200 mana cap with the sphere on.

With the aforementioned Sphere(s) I will sometimes enter a map with 200 of 300 Ability Mana, instead of 300 of 300 Ability Mana. The maximum is properly increasing, but  very rarely the starting value does not.

I can confirm this bug occurs.  Any time you start a map with this sphere equipped, you will have 200/300 Ability Mana.  This includes returning to the Town or Tavern.

It does not prevent you from draining mana to fill your mana bar, and any time a phase change occurs (from warmup to build phase, from build phase to combat phase) your ability mana will refill to the full 300.

Fixed with the Trials Update as Sphere's system was replaced.