I was exploring with one of my pets and did beat one of the red monsters and found a marketplace. Sadly the treasure chest was placed in the market and not collectable.

You can see the issue on my screenshot.

I've had this issue as well, but the chest was on a side which could be reached, and was in fact accessible by trying to enter that corner of the market. Just more data to consider. c:

I have a feeling that this bug is along the same lines of the bug that places a building between you and the rest of the island (which is relatively rare  but not uncommon).

Sadly these 2 bugs together are rather difficult to sqush because of the way the islands are procedurally generated. This is along the sime line of the water items spawning unreachable because they're blocked by rocks/buildings. In order to fix it they'd probably have to re-work or re-write the island generation script.

Without seeing the script that is used to generate the island, I can't say how easy/hard this would be, but in any case, with how rare these occurances are as it is, it's probably a back-burner bug fix that's in the works, but no definite time frame of when it'll get fixed.

Bad luck when this happens, just be glad it doesn't happen often.

the only way i can see this (the chest generation only) being fixed is if the we could loot the guardian for the chest ... as in you kill it the chest spaws there.