Simple hostile mob farm for every thing except spiders. AFK spot 120 blocks above. Mobs travel up water elevator - over a couple of blocks and drop down 25 or thereabouts onto a hopper where I kill them. All fine.
Modified farm to harvest records - water travelling across hopper through 2 high gap with trap door on top so that only creepers can pass through. I stand 5 blocks from creepers and 11 from skeletons trapped behind trapdoor. Skeletons shot at me - hit the creepers. Perfect - all 12 records got within an hour or so.
Modified again to harvest mob-heads - creepers washed out but drop down one block into water on top of hopper. Creepers bouncing around in water. Wait for 4 days (real time) for a thunderstorm - hit first creeper with trident - 6 creepers charged up. Decide to test insitu. Spark the first one with flint and steel and then run back out of range down corridor I made specifically for the purpose. Creeper blows up and takes out most of the building, the chests, the hoppers...
I'm left standing on 3 blocks isolated and have to build my way back to what remains of my afk spot. Now I thought a creeper in water doesn't damage surrounding blocks. What????
Just to cap it all off - no head. May have missed it in gathering up all the stuff from the surrounding environs but damn - that did not go as planned!
Question - if the creeper is bouncing in water and explodes at the top of its bounce would that mean its not in water and therefore damages blocks? I can't think of any other reason this would happen.
Yeah, possibly - Lag..? - effect, like you're describing (and /or Enderman's Block re-placement). "AFK" doesn't mean "Safe," of course, is Kind Of the trick, overall in a Sandbox, "Open World" Game Design, but - sorry, but - good luck, figuring-out the Game Mechanics, without a "security video"
And I don't know why, but I've never fully-trusted the whole "Hostile Mob, 'Bobbing' up-and-down and all-around in Water (or less-often Lava)," scenario. I can still remember though how the Passive Mobs used to "Bounce up-and-down" (on Land), and somehow I actually grew to prefer that (heh) .
Just tried a second time and same thing - having tested with a non-charged creeper and that worked fine, of course...
Anyway I at least got a head this time.
I've attached an image of the setup. Behind the open trapdoor you can see the mobs. The creeprs come down the corridor and into the water. Whichever one is furthest forwards is the one I hit once its charged up.
I'm thinking of rigging up some kind of piston contraption to force them down into the water when I hit them with the flint and steel.
Oh the lower block under the trapdoor is normally removed- just place for extra security whilst I re-build everything.