Undo - does it exclude any fields?

I'm using setRawMetadata in a plugin that allows the user to transfer the value of a chosen field over to another field. Undo appears to work normally, so for example, the user may decide to write the filename to the caption and a subsequent Ctrl/Cmd Z will undo that action and restore the original caption. So I'm pretty sure the plugin is correctly undo mechanism:

MyHWLibraryItem.outputToLog("Transfer between fields")
catalog:withWriteAccessDo( "Transfer between fields", function()

...

However, if the user write the filename to the title field, the undo action "Transfer between fields" appears available, but selecting it fails to restore the previous title.

Any thoughts? Bug?

John