It's either caused by the wisps, or some kind of lag bug (maybe caused by wisps or physics) that applies damage with some delay. I had it happen a few more times, where right after or during combat the game caught up with something and applied several hits in a quick succession, like it recovered from a lag spike.
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Long live 1.1 then. I think it's technically playable, although it is affected by RNG way too much. My current playthrough is the furthest I got in this version, built an exosuit and started botania/exploration. I got a bit lucky with mob spawns, and I managed to run far enough to despawn occasional ghasts. I knew what to expect and built better defenses, so they didn't completely rekt my base, but mostly it was luck. I suppose with the ghast stone breaking fix it's going to improve significantly.
I noticed that blazes no longer drop rods, haha. Good luck finding gold to make golden apples for pre-nether healing. Meteors are fixed, too. I can't find any :'D Definitely don't miss the noise pollution.
I managed to avoid the mutant mobs, so can't give much feedback. I did see some pretty weird stuff...
I think the physics need to be tweaked a bit. They cause some weird collapses in 1.1, the land seems very unstable. Such as this one, as soon as I broke some Fibrous Taint here on top, the below section dropped down. It affects gameplay a lot, a random explosion can collapse your base from below, or the islands get eroded like that causing unpredictable things (this one actually made a gap in the walls of my mining tunnels). That lava phenomenon is probably caused by that as well, my map is full of those.