There is a bug that appears to affect some users running anything later than iTunes 12.9.3.3 (I think) where a change to the media folder location isn't captured properly and the path will reset next time the app is launched. See Make a split library portable - Apple Community to put your library in a shape which uses the default media path with respect to the library file (i.e. a folder called iTunes Media in the folder with the .itl file) so that this particular bug doesn't impact your library any more.
Exactly which steps need to be taken to reach the end point depends on the starting conditions. Assuming you want to use drive D: for your library then it may be best to copy your library files into D:\iTunes, access them there, reset the media folder to D:\iTunes\iTunes Media, then consolidate, and finally clean up the originals stored elsewhere. It depends if there is temporarily enough space to duplicate the entire media folder.
tt2