Sink is controlling shower cold flow

I cannot figure this out ... we got a new hot water heater and now my master shower doesn’t get cold water. The only way to control cold water is to turn on one of the sinks in the master. We’ve replaced the cassette in the shower without any change. My other shower works just fine. I have a video but it’s too large to upload.
When the house was built (we’re second owners), all plumbing was sheet rocked over without access panels. I don’t know where to start ripping out Sheetrock.

Sounds like loose minerals or crud got into the pipes when the water heater was replaced.
And now they are clogging or screwing up some of your faucets.

Change the faucet cartridge in the sink faucet that makes an improvement.

Also, many bath or shower valves have two separate doodads inside, a control cartridge and also a separate pressure balancing module.
You often need to change both.

Where’s the pressure module usually located? Since I changed the cassette already.

If that valve has a separate pressure balancing module, it will usually be found deeper inside the valve body.
Behind the cartridge.

Sometimes it is in a separate chamber in the valve.

Post pics of your valve, the handle and trim, and pics of what exact cartridge you installed.

are you able to shut off the shower water from the shower valve???? when this happens is there equal pressure when turning from col to hot ,when you switch from shower to tub spout is the pressure the same, when you pulled the cartridge did you check hot and cold inlets,use a wet vac on the hot and cold inlets ....