I don't think this is related to search. As far as I know, date range filtering previously worked on Special:Contributions for Flow edits (back when it was done using the year+month inputs).
Also, the "To date" option seems to work correctly, only "From date" is not respected. For example: https://www.mediawiki.org/w/index.php?title=Special:Contributions&target=Quiddity_(WMF)&start=2017-05-28&end=2017-06-03
(Quiddity made a few Flow edits after the "to date", on 5 June and they are correctly not shown. However, we see Flow edits from before the "start date", e.g. from 21 May.)
Since the option to specify a start date is newly added in b66888733c182fd02478c0d351682b15c3f114df (which also changed the year+month input into a single date input), I think Flow needs to be updated to support it. This is separate from the search functionality.
It does not seem to be specific to Flow. As far as I could see there are two issues
(1) When there are no results within the data range, the query falls on the closest available results (?)
(2) When options 'latest' or 'View (older 50) are clicked, the date selection will be discarded. The url might look like: https://commons.wikimedia.org/w/index.php?title=Special:Contributions&dir=prev&contribs=user&target=Yann&namespace=&tagfilter=&start=2017-06-05&end=2017-06-05 , but the result set from 2004 year.
Note: another problem is that result of selecting options for 'latest', 'oldest', etc is confusing and what options were actually selected are not clearly visible to a user.
- There are no results found for June 13, 2017, so the results from June 5, 2017 are displayed.
- the same user as in the reported issue. The user does not have results for the selected date range (May 15, 2017), so the results from 2015 are returned.
CORRECT cases (the user is the same as in the reported issue):
- surprisingly, if the date range includes June 08, 2017, the returned results display dates such as June 06-07, 2017 which makes the previous case (when the results from 2015 are displayed for June 08, 2017) even more puzzling.
- selected date range works:
- one-day selection works too