I am not sure if this bug is with base NWN or Swordflight specifically, but when I load/reload the game my Turn Undead stacks reset or increase. The consequence is that my paladin in Swordflight can pop Divine Might and Divine Protection without much need or thought to conserve. Not that I need those spells every fight, but still.
Turns out neither, it's a default NWN bug dating back to the DE. I just confirmed it with my TotM Blackguard in the EE and my DoD Cleric in the DE. And there actually appear to be two
bugs relating to the application of magical Charisma bonus to your current Turn Undead pool:
1) If you apply any bonus or malus to your Charisma modifier, whether it's gear or a potion or whatever, the UI won't refresh your Turn Undead pool until the next time you use it. So if you have 2 Turn Undead left and you unequip an item, taking your Cha modifier down 1 unit, after using your one of your "2" Turn Undead activations you'll be left with 0. This is rather minor, though I guess it could be annoying sometimes.
2) The game load is miscalculating your Turn Undead pool by adding back the extra Turns resulting from your additional points on top of your natural Charisma modifier. So if you have a base Charisma of 20 from a natural 15 plus 5 bonus from gear, giving you 5 Cha modifier and 8 Turn Undead uses, and you spend all but 1 and then reload, the game will bump you back up to 4 Turn Undead left (the 3 extra you get from the +5 to your 15 Cha). If you only spent 1 Turn Undead, then you get back to 8 because the bug can't take you past your max pool, this is why you thought it was either "increasing or resetting."