I think I've been engaging the cat and turning down the thermostat too soon after loading
I can make mine produce that smell the same exact way. How long are you letting it rip after engaging the cat?
Turning it down to soon after cat engagement and importantly also in my setup... To far. Creates that unpleasant effect. Normally a non issue but I goof it up occasionally. Guessing some setups/installs are simply more sensitive than others.