The opposite length issue

It might be worth querying and seeing if there's any interest. The market for shorter work is expanding somewhat - Tor now has a novella imprint, Prime Books has a novella imprint, Berkley has Intermix, HarperVoyager does shorter stuff (I think) - and since you already have longer work under contract you might be able to interest an agent even with something that short. (Because they'll see you as a career writer, not just a single book.)

However, most of those imprints with the bigger publishers are somewhat... untested. They are less risky than a small start-up, but they are new which means potential sales/profits are harder to predict.

If I were in your shoes, already having longer work published and under contract, I would query this and see what happens.

Yes, it's probably far too short. Too long or too short are both problems. When you veer outside of publisher's guidelines for that type of novel, you're making the road infinitely more difficult.

Length is always a choice, and it's best to know the length a novel should be, and to know this before you start writing, and to hit that length. Otherwise, the road is filled with potholes and roadblocks that shouldn't be there.

It's going to be a question of whether or not you want an agent, I think. There are imprints that pay advances on short novels. Most of them don't require an agent to submit MS (though some like Tor's novelle imprint prefer it).

You won't know if an agent will be interested in the smaller advances from these imprints unless you query. Again, with previous published novels under your belt (from a well-respected smaller publisher) you have a leg up because you're not just querying a first novel but querying your latest work as part of a career.

I'm having the same problem with a 54K fantasy mms. My agent loves it, but can't place it with a print house at that length. Tor's novella imprint might be a good fit, but it's new and unproven. The agent thinks the mms can do better in print. Since I can expand it to 85K, we're trying that route first.

I've encountered the too-short issue in previous MS, so I feel your pain. In my case, it always helped to expand the work. Adding subplots, fleshing out bits where I was too direct. The manuscripts didn't sell, but I was much happier with them at the end of the day, and I definitely learned a lot from the process.

I suppose it depends on how attached you are to the story as-is. You don't want to pad it out like a term paper. Do you see room to expand and still stay true to the story? I think 43k is going to be a tough length for many agents.