There is a NullPointerException that occurs in both the persistent and transient versions of the popTail method in core.rrb-vector. See the tests in the file linked below, in the deftest named many-subvec-and-catvec-leads-to-exception, only the ones that expect a NullPointerException to be thrown.
New failing test cases that fail without the fixes, and fixes for this issue and other found during testing, committed here:
I plan to close this issue as fixed if the build.clojure.org testing results are as successful as they were on my machine.
build.clojure.org test results were all good for these changes, so closing this issue as resolved and fixed.