Posting here because its a more general question. If I have a single ticket with multiple carriers for one leg of my trip, is that supposed to guarantee being able to check bags all the way through?

Had a schedule change a few months prior leave me with the following itinerary: DPS-SIN (3k) / SIN-FRA (LH CodeShare, SQ metal) / FRA-MIA (LH), 3K was able to check my bags through, but used the LH flight number for SIN-FRA. I got to the transfer desk at SIN and had I not asked about my bags, they'd probably be in purgatory. SQ claimed they don't have an interline agreement with 3K, and insisted I needed to go landside to get my bags; only when asked where to collect them did the agent realize they wouldn't ever go on the carousel and magically got them picked up by SQ and put on my flight.

Guessing this isn't a normal occurrence, and curious in general if interline restrictions would normally prevent a ticket like this being created or can TAs create these tickets where bags cannot be interlined?