This is probably an easter egg.

What's with the BIG egg in Blast from the past?

Remember, it has to be Wednesday. Go to these maps again. Maybe it will trigger the achievement.

@Wattras thx! Will try to disable steam cloud. The first solution is not working for me…

@lazionalista
Maybe disabling "STEAM CLOUD" in the game will help. Maybe.

The game is bugged. Sometimes this solution helps.
1) Right click on Lumencraft in your library and select "Properties"
2) Click on "installed Files"
3) Click on "Verify integrity of game files"

You are not the only person who has this problem.

Killed all four frogs, but no achievement… Is it bugged?

so to get these i needed to:

1 - build a repair gun and RNG the lab to get lumen erosion
2 - build max drill and a lot of repair guns. stop flame & dig my way down. probably died about 10 times on this one as was by far the hardest to actually get to. the frog just died of lava when i died there.
3 - this one was hard to find but i used dynamite which seemed okay. built a pistol and max drill. The drill seems to get through some of the pipes as well but i just used 3 dynamite and then just shot him
4 - just ran to the end after they broke through and threw them at the frog

Update:

I could handle it, i had to repair the Game files and restart the System. After this it suddenly takes damage from the flares.

Thank you for the Guide, with those Pictures it´s very easy to get the it :)

greetz Reaper

I don't remember. It just happened. Maybe a flare, maybe an explosive barrel. Sorry. I forgot.