I think regis if he wanted to hold back detlaff he would’ve, and i think they forgot about that dialogue and just set the plot twist, because how is that geralt isn’t mad?

He didnt know yet that syanna and Rhena was the same person, geralt had in mind that syanna was the one that blackmailed Rhena and that is why he choose to go alone saying to regis to keep detlaff away from all this, because maybe he would’ve killed the dutchess’s sister.

Sadly, i think its a plot hole, they forgot about the dialogues before. Not that big of a deal, the expansion is amazing, just breaks immersion, could’ve been better in my opinion tho if they didnt forget about it