IIRC, this was changed back in Nov. 2020, Patch 3.4 (Outcast Engineer patch) (as discussed by commenters here: Barrels After Patch 3.4). After that patch, (on Legend+) I’ve never seen an explosive barrel spawn in the market area and on the ground floor of one of the houses just before the market area. All barrel spawns in those areas (or in all of the map) have been made fixed. I believe this was to make the challenge less trivial and not have three explosive barrels spawn near the cart event.