In one photo album, "Keep sorted by title" doesn't seem to work

Photos distinguishes between the Title of a picture and the Filename. It sounds like your pictures have Filenames but no Titles. If there are no Titles, and you ask Photos to sort by Title, then it will sort by date (in your case, the time the slide was scanned), because all the pictures have the same (blank) title. I'm not really sure how they're ordered if dates and titles are exactly the same, as it seems in your case.

So if you want to control the order of the pictures, you need to give the pictures titles. I do that in two ways-- if I want to add titles before I import the pictures, I do it with GraphicConvverter ($40) which has all manner of batch operations. Mostly I use the app Photos Workbench ($30) which works within Photos on the System Library and can add titles with lots of different formats of your choice. A possible Title dialog looks like this:

You can include an Index starting with whatever number you like, and you can enter text like a category that's the same for all. You have lots of choices

and more.

I usually use a date, index, and category type title like: "23 08 15 - 027 - Swimming in the Lake." Then "Sort by Title" will put them in date order, and pictures on the same day will be in the order I want.

Workbench also has Find and Replace, so when you spell Swiming wrong, you can go back and change all occurrences. The company seems very responsive.

There are also scripts available for free, written by reliable users from this site, that automate title changing, but I find Photos Workbench to serve a number of useful functions.

Which system version is running on your Mac?

Sorting by the title is working differently, depending on the system version. On macOS 10.14 Mojave the filename would have been used as a default title when sorting by title, but on all more recent system versions Photos is no longer using the filename as a default title when sorting by title.

You can see, if a photo has a title, when you enable this option in the View menu: "View > Metadata > Title". And the aspect ratio button needs to be set to display the icons in the correct aspect ratio, not as tiled squares. Then photo will display the titles below the thumbnails, when viewing an album. The title will be dimmed, if Photos is showing the filename instead of a title, and the dimmed titles are not used when sorting by the title.

The dimmed, gray titles are the ones, where you would need to open the Info for the photo and copy and paste the filename to the Title field.

The aspect ratio button is in the toolbar, next to the size slider:

Select all of the photos in that album and use this AppleScript (compiled as an app) to enter the file name into the Title field: Script: Changing the Title to the Filename without Extension

Then the photos should be sorted alphabetically by Title (as long as it's an album you created).

A good question - I have not yet tested this. I'll try to create a small library for testing.

I had a lot of fun with Monster Park, when it first came out in 2018. I took my Dinos for a walk, wherever I went. 😀

léonie, do you have an idea of how Photos sorts pictures by title when the titles and dates are all the same? Is it import time?

Love your Dinos...