I have the same - I added photos one by one into Shared Album:
As Old Toad mentioned, I can sort them in my local shared album with the View ➙ Sort menu and selecting "Keep Sorted By Newest Date Added" and all photos are in the correct order: the first photo I added is in the top of the album, the last photo I added is in the bottom of the album.
But in online sharing link the photos are in reverse order.
So, I used "workaround" - I started adding photos to shared album in opposite order.
For now, I have shared albums with not so many photos and no idea what to do if I want to share albums with hundreds of photos... Manually adding photos one by one is not exactly how I prefer spending my time :)