I unfortunately got bit by this yesterday.
I have one UA ticket SFO-NRT and a second QF ticket (AA J award, FWIW) NRT-PER.
On the outbound, with a four-hour NRT connection, UA unquestioningly tagged my bag SFO-NRT-PER. On check-in at NRT, I gave the QF rep my UA bag tag numbers and all was well.
At PER, on the other hand... agent informed me no interline checking outside of Oneworld or if on separate PNRs. She said that if I'd been on AA out of NRT (even on different PNR) it would have been acceptable -- but not in my case with UA.
I had 8 hours at NRT anyway and had planned to check into a Hilton NRT day-room, so going to baggage claim first wasn't that big a deal, though I found the whole thing disappointing.
To QF's credit, my priority-tagged bags were two of the first four off the belt and there was *no* wait at baggage claim -- they popped right up.
But still: in an era of degenerating air travel, interlining always felt to me like a nod to the olden days and airlines working together. Not so much anymore.