Sounds like it was just a small section of incorrectly mixed (possibly contaminated) filament at either the beginning or end of a production batch. When they start mixing the plastics there can be some irregularities in the flow of materials and you get a bit more of one thing and a bit less of another. That’s normally cut away and discarded once everything is flowing correctly. Sometimes it’s missed or they didn’t cut enough.

I’d consider it a one off, or very rare incident, and not a general overall quality issue unless it starts happening regularly.