Mods:

• Lost Cities: lostcities-1.12-2.0.9
• Lost Souls: lostsouls-1.12-1.1.4
• Biomes O'Plenty: BiomesOPlenty-1.12.2-7.0.1.2394-universal
• Minecraft: 1.12.2
• Forge: 2708

The other day when playing I kept noticing a strange issue where my world would look like the seed had been changed and the world was generating differently. When I recreated my world at that point I noticed that it looked like the newly generated content from my old world belonged, but if I completely restarted my game and used the same seed again the content from my original world was generated instead. (Example)

When I was generating new worlds to play on I also began noticing that most of them started to look exactly the same, and noticed that I would start in the exact same biome all the time until I restarted my client.

I put 2 and 2 together and with a bit of trial and error after removing all of my other mods I basically noticed that something weird seems to be happening specifically with Lost Cities (BOP) generation if you load a world and then create or change to a new one.

Images:

I made a world with Lost Cities (BOP) seed 966759076
I made a world with Lost Cities (BOP) seed 12345, then deleted it and used seed 966759076

Have same problem. After some hour if i genered new map with same seed, then map is totaly diferent.
If i start play on any map, then after some hour if i load my map and go to new chunk, then this chunks are from another seed - stone barier atc.
Seed isnt stable and is change during playing map (unstable seed)

I tray on FTB DW20 1.12 modpack 2.3.0
lostcities-1.12-2.0.10.jar
BiomesOPlenty-1.12.2-7.0.1.2399-universal.jar