SOLR-7631 and SOLR-7605 I became fairly confident that the only explanation of the behavior i was seeing was some sort of bug in either the randomized codec/postings-format or the UninvertedReader, that was only evident when two were combined and used on a multivalued Numeric Field using precision steps. But since i couldn't find any -Dtests.codec or -Dtests.postings.format options that would cause the bug 100% regardless of seed, I switched tactices and focused on reproducing the problem using UninvertedReader directly and checking the SortedSetDocValues.getValueCount().
I now have a test that fails frequently (and consistently for any seed i find), but only with -Dtests.codec=random – override it with -Dtests.codec=default and everything works fine (based on the exhaustive testing I did in the linked issues, i suspect every named codec works fine - but i didn't re-do that testing here)
The failures only seem to happen when checking the SortedSetDocValues.getValueCount() of a SlowCompositeReaderWrapper around the UninvertedReader – which suggests the root bug may actually be in SlowCompositeReaderWrapper? (but still has some dependency on the random codec)