I have been trying to find the link for so long regret not bookmarking the moment I watched it, but there was a (?dev?) interview, supposedly we could potentially have revisited the hells more specifically Vlaakith's temple grounds which perhaps would have fleshed out more of the detail we are having on this topic maybe (?) never came to be beyond the stage of concepting I assume a lack of time was probably the biggest reason to it, that subject included a lot more that also didn't make it to the game for reasons such as not matching the flow of the game and lack of proper tech to have other details work practically from a gameplay perspective regarding other stuff, like something that could take place between Act 2 and Act 3 for example.
I keep getting the impression that Mephistopheles did not necessarily know about the theft, more or less having the same thoughts like what @fylimar is expressing above me, a revisit to frozen region of Meåhistopheles domain could have further developed a new story branching off of Baldur's gate 3' story regarding this subject, imagine that.
