Both cranks should be the same where they attach to the hub.
The pedal thread must be opposite. So both the crank & the pedal
must be left-handed (tightens counterclockwise) for the left side
of the unicycle.
As the wheel goes around, the crank does a full rotation around
itself every time the wheel does a revolution. But the pedal always
faces up. So while the righthanded pedal will tighten more, a
right-handed pedal on the left side will loosen.
But if your problem is with the crank going loose, not the pedal,
it could be that the extra movement on the loose pedal loosens
the crank, or it could be an unrelated problem. So, read all the
other posts that you received in this subject.