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PC EU Megaserver
Minie Mo - Stam/Magblade - DC
Woody Ron - Stamplar - DC
Aidee - Magsorc - DC
Notadorf - Stamsorc - DC
Khattman Doo - Stamblade - Relegated to Crafter, cos AD.
Xbox One | EU | EP
Beardimus - VR16 (810CP) Dunmer MagSorc [RIP MagDW 2015-2018]
Emperor of Sotha Sil 02-2018 & Sheogorath 05-2019
1st Emperor of Ravenwatch
Alts - - for the Lolz
Archimus - Bosmer Thief / Archer / Werewolf
Orcimus - Fat drunk Orc battlefield 1st aider
Scalimus - Argonian Sorc Healer / Pet master
Fighting for : The SAXON Guild
Fighting with [PvP] : The Luna Wolves
Small Scale [PvP] : Random Reds
Trading Guilds : TradersOfNirn | FourSquareTraders
Xbox One | NA | EP
Bëardimus - L42 Dunmer Magsorc / BG
Heals-With-Pets - CP160+ Argonian Sorc PvP / BG Healer
Strange. There's nothing in Battle Spirit that affects magicka. The bonus to health is the only stat change.
You might look at where you are getting your magicka, aka gear, skills, passives, and buffs and checking which one is getting changed when you go into PVP. Something about your build is changing between PVP and PVE, but it shouldn't be an affect of going into a PVP area, especially in CP PVP.