I made it to Vulcanus a day ago and just noticed when I was playing today that I could only find remains of Demolishers. So I went far off and found Behemoth ones. They were still alive, but receiving constant damage they were able to survive due to their high regen.
Looking around and googling it.. supposedly its due to "This behavior is exactly what happens when the global Itaverax passive environmental damage aura is accidentally or intentionally leaking into other planets like Vulcanus".
At least thats what the LLM thinks.
I did try disabling mods on and off and only find that disabling Abacayba by itself, stops whatever DOT is effecting and killing the demolishers on Vulcanus off.
I've only been to Abacayba a bit, and had to reload a save before I went there as I couldn't handle it at all yet. But while I was there. I saw no on going damage like that there.

thanks for the repport, will take a look latter

Thanks! I'm on 2.0 by the way. Also, thanks for the mod, I do like what I've got to mess with it so far. I'll eventually go back to Abacayba when my damage research is high enough to actually survive more than a second or two.

i will try to reproduce that in 2.1, also, send your factorio-current.log, so i can check if its some mod-mod iteration

I am running with nearly all planet mods right now and ran into the same issue. I made a few series of logs etc and the damage I find runs back to this mod. (most likely) I am also running ERM mods (all of them) and a few other mods that may affect Demolishers. But this to me really looks like the smoking gun, the melee damage increase is being applied to the normal demolishers, where they in my case deal 1250*3 itaverax damage per tick to themselves. It seems to be one specific segment hitting the segments infront and behind?

When I enable a damagelogger script for example I get this:
5427539 | type-impact dmg=0 cause-big-demolisher health_after-191650.078125
5427539 | type=itaverax dmg=1250-cause-big-demolisher health_after=190400.078125
5427548 | type-impact dmg=0 cause-big-demolisher health_after=259850
5427548 | type=itaverax dmg=1250 cause-big-demolisher health_after=258600
5427539 | type-impact dmg=0 cause-big-demolisher health_after-188300.078125
5427539 | type=itaverax dmg=1250 cause-big-demolisher health_after-187050.09375
5427548 | type-impact dmg-0-cause-big-demolisher health_after=256500
5427548 | type=itaverax dmg=1250 cause=big-demolisher health_after=255250
5427539 | type-impact dmg=0 cause-big-demolisher health_after-184950.09375
5427539 | type=itaverax dmg=1250 cause-big-demolisher health_after-183700.09375
5427548 | type-impact dmg=0 cause-big-demolisher health_after=253150.015625
5427548 | type=itaverax dmg=1250 cause-big-demolisher health_after=251900.015625
5427539 type-impact dmg=0 cause-big-demolisher health_after=181600.09375
5427539 type=itaverax dmg=1250 cause-big-demolisher health_after=180350.09375

im working on a new mod and TDSrock will release a fork to fix the issue

https://mods.factorio.com/mod/itaverax-demolisher-immunity

This tiny mod fixes this specific problem for now untill Emerson can implement a proper fix. Once a proper fix lands I will depricate this mod.