It;s only a few minutes work, so attached is a replacement for the file /admin_trees_sanity.php. Just replace your existing one.
This will be included in the next release.
For anyone interested (who perhaps have other checks they’d like to add) the reason this was simple is that its a straightforward comparison of dates between two basic GEDCOM level-1 tags (BURI, DEAT) within a single record (INDI). The hard ones are those that require checks at a lower level, and / or cross record types, such as comparing birth to marriage date for example. One is an INDI, the other a FAM record.