1EdTech Online QTI validator vs free validator

We have several customers (schools, districts etc) that use QTI import in itslearning to upload tests and questions from other sources. Sources could be big and small content vendors, or content they created themselves and exported from other tools. From time to time those imports fail, and we get support requests about them.
We comply with QTI v2.1 (certificate expired, I know, working on it). The sources of the import files most often come from tools that are not certified but claim to comply.
Since our customers are not 1EdTech members, they cannot access the validator, but some of them try using this: https://webapps.ph.ed.ac.uk/qtiworks/anonymous/validator
However, we now see that there are packages that PASS the free validator but FAIL the 1EdTech validator.
Does anyone have insight into what the differences are, or otherwise any input as to how our customers can check the validity of packages?

Frøydis Hamre
Product owner, itslearning