So, I just modified an MKV file's description and wrote back the changes to the metadata. I now access the {File.MetadataWriteBack} variable, which still gives me a date from back in 2022. On the other hand, {File.MD.XMP::xmp\ModifyDate\ModifyDate\0} correctly reports the current date as today. So, why isn't the new variable making note of the change that I just made to my file?
The variable {File.MetadataWriteBack} is filled when you write back a file for the first time.
I've just tried it with some RAW and JPG files and it works just fine. After the write-back has completed, the variable reflects the current date and time.
IMatch tries to sync the write-back value in the database with the "last modified on disk" to avoid confusion. What does {File.Modified} show for your test file? What does Windows Explorer show for "last modified" for that file?