MacOS – Deleted photos from MacBook but storage space not reclaimed

It's likely in /Volumes/MobileBackups and you can easily delete it by entering Time Machine and navigating to the folder where the file existed. Select the file then command-click (right-click) and delete all backups of that file.

Normally, the system information window shows the backup space, but I have seen this window get hung up and not properly report storage space when Spotlight and/or the disk catalog are wonky.

You can also wipe all local backups by disabling the local store:

 sudo tmutil disablelocal

If the above steps doesn't work, you'll want to boot into recovery mode and run Disk Utility to check that the directory structure and file system catalog are correct. A fsck -fy / in single user mode is also equivalent if you are so inclined to fix things from terminal or want to perform a safe boot and have the system clean things up systematically.

Two things to note:

1. the small print under "Optimize Mac Storage" states that "Originals will also be stored on this Mac if you have enough storage space", so hopefully the fact that you have plenty of space to spare will qualify as "enough storage space" and nothing will be deleted immediately (hopefully!)
2. even though you're not offered the option in the dialog, once you select "Optimize Storage", you can immediately switch the radio button back to "Download Originals to this Mac" Neither of these things necessarily guarantees that the originals won't be removed and re-added, but at least you will end up with all the originals on your Mac once it has had a chance to synchronize everything to its satisfaction.