When an existing filter tags emails to a certain tag, then filter assistant
should offer merging when another email is manually tagged (during filter
assistant active) or when "quickFilters: Create Filter from Message..." is
selected on an email with same tag.

This behavior would be consistent with the same behavior for filters that move
emails to other folders. (merge is suggested if an existing filter moves emails
to the same folder)

Created an attachment (id=8476) [details]
3.6 prerelease 24

Quick bugfix that should evaluate the tag properly.

Note that merge is only suggested if the tag is matched with the first
(primary) action of the existing filter.

Fix released 16/06/2018 in version 3.6