Photos change sequence in my album.

When I do a custom sort, it sticks. But it is very vulnerable to mistakes and accidents. I know this for a fact!

I am guessing that you want a particular order to tell a story-- is there another reason that your pictures are not in order? If there is a date/time thing, then that can be fixed pretty easily.

For a custom order I rely on Photos allowing me to sort pictures automatically by Title or by Date. If I have a set of pictures for which Date doesn't do what I want (which seems to be the usual case!) then I add maybe temporary titles to help keep the order until I can do something better. Another trick is to take a screenshot of the thumbnails showing the order, in case I need to re-adjust.

Once the pictures are ordered the way I want, I use two methods to keep them that way.

#1 I change the Titles with the 3rd Party app Photos Workbench ($30) that lets me apply incremental numbers to the titles to keep them in order. You may find a (free) script at the above link that does something similar.

#2 If I don't mind making up dates, I change the dates to keep them in order. The easiest way to do that is to use a script that you can find on léonie's page here:

Thematic Index to Photos for Mac related … - Apple Community

called

Script: Batch Change the Date and Time to a Fixed Date

that uses the first picture's date and time and increments the following ones by a minute or whatever.

The disadvantage to relying on the Titles is that it doesn't work in iOS -- iPhones and iPads don't order by titles. So, I usually do titles first, and then, once satisfied, I change the dates and times.

What kind of album is it? A standard album or a Shared Album? Or perhaps a slide show project?

The custom order of shared albums can change, when new photos are added or when someone adds a comment to a photo.

The custom order of a slideshow project can change when we add new photos, because Photos will automatically rearrange the photos to fit them to the templates.

For a standard album it helps to add a running number as a prefix to the title of the photo. Then we can sort the photos by the title to preserve the custom order.

To make the manual sorting easier, it also helps to use a folder with several smaller albums instead of one large album. I am creating a folder with a separate album for each day of a trip to sort and weed out the photos. Sorting is much easier for small albums, because we can see all photos at once and the distance for dragging is shorter.