[Faceman]

if Jaheira's high level incarnation has only 14 DEX that should explain it!

He has finished the game with his PC so he obviously is high level. The game checks for your PC's level when deciding which incarnation of an NPC to spawn as joinable for your party.
So if his PC is e.g. a lvl 17 mage the game would spawn lvl 12 Jaheira with only 14 DEX.
Solution would be to either shadowkeep her or if you want the low level incarnation (to be able to decide more on her character development) you should start a multiplayer game with a low level PC free Jaheira and have her join. Then kick out the PC and substitue him with your saved one. Now do the multiplayer trick and port the game to single player.