Just wanted to put this out there and see if any other schools had observed something similar with regards to third party tool behavior in the new gradebook.
It seems to be that late policies applied externally within the tool (eg, Macmillan, Pearson) for assignments with a submission type of External Tool (eg, Kaltura), the late policies don't get transferred over onto the Canvas side. Late policies only apply to submissions of the Online type.
In other words, Canvas doesn't "know" that when a grade is coming in for Assignment 4 for Joe Student, that that grade has a "late" attribute applied to it, so it doesn't know to apply the internal Canvas late policy to it.
We're currently mitigating future issues by writing up some documentation for our faculty, but I wanted to reach out and see if anyone else found this behavior in their testing.