I think it can happen when they have a cold too. My baby did the other week when she had a cold.
My boys nappies are full of mucus but he has had ongoing colds for weeks on end (joys of a germy toddler sibling) so I just presumed it was post nasal drip and the mucus going into his stomach and then passing through.
My baby's were worst last week when she had loads of drool from teething, the past few days she has been drooling a bit less and her poos are less mucusy.
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My girl has that now but she also has milk allergy where her stool turns like that. So I'm not sure if it's something I ate or from teething, but teething definetley can change it to runnier 🙂