To add more troubleshooting, I'm not getting any results for some users (but am for others) when querying the revision_userindex table on Quarry, so this may not be just an API issue: https://quarry.wmcloud.org/query/59089

It could be that I'm missing something completely obvious, so thanks in advance for your help!

ucdir=newer does the job, according to the docs

https://en.wikipedia.org/w/api.php?modules=query+usercontribs

ucdir
In which direction to enumerate:

newer
List oldest first. Note: ucstart has to be before ucend.
older
List newest first (default). Note: ucstart has to be later than ucend.
One of the following values: newer, older
Default: older

https://en.wikipedia.org/w/api.php?action=query&format=json&prop=revisions&list=usercontribs&meta=&uclimit=10&ucstart=20100601000000&ucend=20210601000000&ucuserids=13286072&ucdir=newer

Or swapping start/end
https://en.wikipedia.org/w/api.php?action=query&format=json&prop=revisions&list=usercontribs&meta=&uclimit=10&ucstart=20210601000000&ucend=20100601000000&ucuserids=13286072