From this it sounds like you only looked at it from the 'non-invasive' cap-off view (or perhaps also poking within it with a pin or something, which I'd also done and not seen burn from that very limited angle). Rather, the filler and wicking material needs to be completely extracted and unrolled to know for sure whether burn exists within. So, the cartos that is done on are sacrificed, no additional refills in the wings for them.
If one wants a meaningful answer as to what may lie within, sacrifice of at least several will also be necessary. (vaping style varies from person to person of course; and certain situations e.g. being busy or otherwise tied up may lead to occasional pushing of envelopes on switching out or topping off; then for some there are factors such as HV mod or pass-thru use; and regardless, cartos themselves vary too. Not just reg vs. premium, but due to QC issues, some are just poor performers which among other things may unexpectedly 'behave emptily' sooner than others.)
This post is intended to raise nothing more than the fact that only complete disassembling (of multiple used cartos) will shed any light for a given person.
(Actually, all may not be lost even with those disassembled cartos. Appears that ppl are starting to look deeper into the feasibility of dripping with them with filler removed, and Dogsbody will apparently be doing a video on it soon. Not to say that would be of interest to everyone but throwing it out there as something to follow for anyone who might be.)