Been seeing this a lot lately. Always happens when dewinterizing.
The problem typically occurs when winterizing by draining the hot water heater through the low point drains in the fall. This allows crud from the water heater (bits of anode and calcium) to enter the low (cold water) inlet to the water heater as it drains.
When antifreeze is reintroduced it can push this crud into the valve body of the faucets and/or the toilet flush valve. While it may not be noticed in the winter (faucet not open long enough), it will definitely be noticed in the Spring.
Getting rid of the issue (as noted in previous posts) can be a real chore. I discovered my error when I had to repair my toilet flush valve. Repairing a Thetford Aqua Magic V
I changed my winterizing/dewinterizing procedure to close the heater valves FIRST when winterizing (then drain through the anode/outside drain plug); and open LAST when dewinterizing (allowing it to fill with clean water).
Prevention is way easier than the repair.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW