You need to clear beamExceptions also.
\set Timing.beamExceptions = #'()
Most automatic beaming settings for a time signature contain an entry
for beamExceptions. For example, 4/4 time tries to beam the measure in
two if there are only eighth notes. The beamExceptions rule can
override the beatStructure setting if beamExceptions is not reset.
This has been added to the doc by Carl very recently.
Did you have a look at latest version of the doc?
NR 1.2.4 Beams> Setting automatic beam behavior
http://lilypond.org/doc/v2.13/Documentation/notation/beams.html