I am not very optimistic, that this will be fixed. Shared albums could never be sorted. It has been this way since the shared albums first came out. Originally they have been called Shared Photo Streams, and the original name made it clear, that they are streams of photos and not albums at all. They have been renamed to Shares albums with iOS 8, and now the users are expecting them to be working like albums, but they are still the old streams.
But it is clearly a bug, that multiple selected photos get rearranged, when adding them to an album or shared album. At least it not user friendly behaviour.
You may want to send feedback to Apple with a feature request. The feedback form is here:
Photos for iOS and iPadOS - Feedback - Apple