Stopped getting chests from some(near half) 1st bosses kill after soft reset, no chest sign near bosses location. Now cleared lvl 60 and got chests just from lvl 5, 25, 30, 40, 50, 60

You only get a guaranteed chest for the first time you kill that level boss. On repeat clears, the chance is only about 25%.

Do a run of the second campaign, it'll give you another batch of guaranteed chests.

second campaign s every campaign other then the very first one which was to reset the world.
you have a 20% chance of a chest on any boss level you have ever previously done, and 100% on any that you have not reached, for example perhaps you never got to 150, well there will be a chest there...