In a professional capacity but not medical I have recently had limited contact with a family where FII is suspected but as you say, most information comes from parent reports.
These supposed medical events happen when the child is at home, the only evidence is from the parent, maybe another family member who had been told ‘Jonny had a seizure’ and repeats this as fact but didn’t actually see it themselves as they would never suspect the other parent of making it up, but if a parent takes a child to hospital saying they had a seizure a medical record is made, then there will be a different medical issue, breathing problems, feeding problems, mobility issues, suddenly the child has multiple medical teams doing different tests and assessments and trying to unpick what is true and not becomes difficult and often these teams do not communicate with each other.
Then there is the reluctance of statutory agencies to make an allegation of FII in case they are wrong, and, whether it’s a social services role or health role to investigate, in the case I came across the family moved frequently, accessing care in different places, just as suspicions were heightened enough that investigations might have started the family moved out of area so ‘it’s not their responsibility anymore’, parents are able to easily manipulate the system and children get lost as no one wants to deal with such complex cases where nothing is proven and the impact on the child either way is going to be traumatic.