Thanks for confirming this isn't an issue on your end. Was enough to commit to a full test and in doing so reveal the source of the problem.
Basically I already had this conversation with him immediately upon entering ACT III (the camp site on the cliff overlooking Rivington), but due to some broken spaghetti code of his this pop-up dialogue is indefinitely stuck-in-queue to trigger once again the next time he finds himself outside the camp, which in this case ended up overriding my Mystra dialogue because I haven't taken him out since the beginning of ACT III, so the bug has been present all this time and only revealed itself in the worst possible moment.
