So AAA-BBB (eticket 1), BBB-CCC-BBB (eticket 2), BBB-AAA (eticket 1) all in 1 PNR.
Nested return is not that unusual with airlines or travel agents that sell these type of tickets.
And can be if different GDS like amadeus & sabre.
The other airline may also a PNR that you do not see (or are aware of). PNR is just the plain manilla folder that hold the info.
Protection will depend on the carriers & ticket t&c's. I would expect protection.
For example AA would not interline checked baggage, as per the defined AA interline rules on separate tickets. But QF would as interline on the same PNR
There is no 1 simple answer. Will depend on the exact circumstance and ticket t&c's.