The field name in MC must exactly match the tag name in the file, including Case, Spaces, Underlines, and everything else. Then MC should import the value.
So is your tag called "ARTISTSORT", or "ARTIST SORT", "ARTIST_SORT, "ArtistSort", "Artist Sort", "Artist_Sort", "artistsort", "artist sort", artist_sort", etc.. You get the idea. Check inside the files, not via dbPoweramp or any tool that interprets or formats tag names.
If this worked in MC25 but doesn't in MC26, check that the tag created by dbPoweramp hasn't been changed in some way. Use a HEX editor to look inside a file that has the correct tag, and one that MC hasn't picked up the tag.
Also note that tags are handled differently for different file types. That is, FLAC files are handled differently to MP3 files, as are ALAC files, WAV files, etc. Are you ripping to the same file type that used to work before?
Also, make sure you are running the very latest version of MC26. There were some fixes around tags a while back, although I don't think they would have impacted an "Artist Sort" field.
If it still doesn't work for you, come back with some more information, such as the field name used in MC, the tag name used in dbPoweramp, and what is in the files, exactly. Plus describe your workflow for ripping, tagging, and importing into MC.