ADD sometimes doesn't get written to chunks if the server restarts while in the middle of world generation due to the freeze timer being too restrictive. This is most visible when the server enters automatic freeze recovery while generating sections of the nether.
The most visible side effect of this bug is that blocks added to the nether worldgen system which have block IDs > 255 will not be correctly saved. As a result, it looks like random blocks of creosote/coolant are just all over the place in the newly generated chunks.
I'm not really sure why only the ADD ends up being corrupted while everything else is still fine. This is kinda baffling to me. Probably shoddy implementation of the map saving system.
A potential workaround to avoid the problem is to set a really long freeze timer, which would prevent nether generation from triggering freeze recovery. Another possible workaround might be to somehow reset the freeze timer during world generation, although this might require a more extensive rewrite of the chunk generation code.