Is Interlining supposed to be guaranteed on a single ticket?
 

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?

Was the trip in the direction on one single ticket number, or was it multiple ticket numbers on one ticketed PNR?

A travel agent PNR can be composed of multiple airline PNRs. But at least for the major online travel agencies that I use in North America and Europe, if it’s on a single ticketed PNR, then the bags generally can be thru-checked as long as the layover isn’t too long.

Could it be a case of an ill-informed or presumptuous transfer desk agent who is more used to seeing 3k passengers who have separately ticketed PNRs?

In this case it was one ticket number, issued by LH on LH stock. Originally it was 3K-LX-LX but due to cancellation of the middle leg, it got rebooked to 3K-SQ-LH

Interline ticketing agreements and interline baggage agreements are separate things. Two airlines that have one usually have the other, but not always.

In your case though, it could be that the two airlines providing you transportation don't even have a ticketing agreement. Becasue your ticket was isued by LH, it could be that LH has an interline ticketing agreement with 3K and SQ doesn't.