A/C cold only on passenger side; barely cool on driver's side

There are no humidity sensors, in winter the AC compressor works to cool down the air "this will remove humidity" before heated.
in the case of broken flips in winter: one side will blow warm air "less than usual" and the other side will blow cold air.
You need to remove the dashboard then remove the AC system and fix the flips.

 

You sure there are no humidity sensors?

I found my a/c only worked on the passenger side (mine was a RHD car) when the a/c gas was low. Once it was topped up the a/c was cold on both sides. A/c loses about 10% a year, so a 5 year old car may be low on gas. Much more likely than jammed vent mechanisms.

If you are re-gassing a Prius you MUST ensure the correct non conductive lubricant is used or expensive damage will be caused. Not all a/c re-gassers have it.