Installed a new moen 1225 cartridge as the old one had seized and was not pressure balancing. New cartridge installed smoothly but when toilet flushes anywhere in house shower occupant gets scalded still won't pressure balance?

Question, what type and size are the water pipes? What is the house P.S.I. This sounds like more of a piping problem rather than a cartridge problem. Back to you , Tom

Problem is with pressure balancing valve, not the cartridge.

See this parts break down.

https://www.moen.com/shared/docs/exp...ews/3170pt.pdf

Call 1-800-BUY-Moen with model number and request an new pressure balancing spool.
Warranted for life.
You could try soaking old one in vinegar to remove mineral deposits.