No barrels with HapisIland only with Oxide

Missing barrels on "HapisIsland" maps

If i delete Oxide - barrels are spawned

PS: only HapisIsland

 

I didn't have barrels on my Hapis server either for the whole month I ran it... Though it was the map itself that was messed up, so I'm surprised you say they do spawn when you remove Oxide. In fact, it's not entirely true, barrels did spawn on my Hapis server but only at the airfield and at the rad town near the lake.. NONE near roads or other rad towns at all.

You can use Cornucopia to force barrels to spawn all over the map, but they won't be tied to monuments/roads, it'll be random.
http://oxidemod.org/plugins/cornucopia.1264/

As a reference I ran a total of 500 barrels on Hapis. It's way more than what spawns on a procgen 4k map (more like 100ish) but Hapis is such a mountainous map you'll get tons on the high peaks so 500 won't feel like that much.