Hmmm - you can't do a regular search on it either, e.g. EPUB file date before Jan 1 2020; but you can sort on it.
I'm half-inclined to regard that as a 'bug', I'll PM the Kovid and chaley and ask them to chime in.
Meantime
maybe this plugin can help ==>>
Save Composite Custom Columns - it runs when the library is closed, but I'm not sure where it saves the composite (built from other columns) cells.
You could try copying the column to a regular date column using Bulk metadata edit->Search and replace - but if you edit an epub in any way the template and copied columns will get out of whack.
BR