There are no limitations on the size of individual FLAC comments (actually a 32 bit size). There is a 24 bit (16mb) size restriction on any FLAC container section which effectively limits the overall size of the tag area (not counting artwork) to ~16mb. Every artwork item is limited to ~16mb. It is somewhat unfathomable that the tag area could overflow unless dbPowerAmp is converting artwork to the older embedded base64 format. The only thing that should be possible is that an individual artwork item is greater than 16mb and has to be dropped. In eight years I've seen that once.

It seems to me that unless artwork is dropped you should be okay. If you still want an action I can put something together to do a type of diff. Let me know.

There's a new action on the web resources page named Simple Diff. It was built on Yate v6.0.2 which was released today so you'll have to update in order to run it.

The action is somewhat a cheat. I started writing a diff when I remembered that Yate already has the ability to present changes from the current state to the initial state. In the action you load and select two files. I copy all the metadata from one to the other; get the differences and then restore the file. (nothing is ever written). The difference are displayed in Yate's Log Viewer.

Yate has an Unhandled metadata panel. This is a bucket for items which we either don't handle or out-of-bounds items specific to particular audio formats. It's possible that the conversion processed some of these items (although I doubt it). However, as these items are not checked by the difference logic, I'm dumping them to the displayed information so that they are evident.