Hey Aimee, this is the perfect place to ask this question!
I’m wondering what your cough is like. Is it a dry and irritated cough or a wet cough with mucus coming up? If it is wet, does the cough end once you pass the mucus? It sounds like there is mucus being released but I just want to make sure.
One thought is that the tiny hairs from the mullein are irritating your throat and provoking the cough. It’s important to strain the mullein (through a cloth works best) very well to make sure the hairs don’t get through to your tea.
Or, it could be that the expectorant action of the mullein is working to loosen catarrh, as you suspected.
Please let me know what your cough is like and if you’ve been combining the mullein with other herbs in your evening tea and we can go from there!