More than likely it was probably priority and your filter was running after ACF. Before ACF runs all you’ll get is an array of IDs so that makes sense. If you run it before ACF then you’d basically just want to add IDs, for example a default image in each gallery.
The documentation on the site is the best you’ll find, for the most part. Unless you find a tutorials by other people. I’ve figured out most of what I know by trial and error and doing a lot of echo somethings and print_r(something).
The main thing to remember when creating filters is that ACF uses the same hooks to do the work that are in the documentation for us to use and 99% of these happen at the default priority of 10. What you’ll see a <10 will be significantly different than what you’ll see at >10
