Baking NOT required.. and crushing should NOT be too small as it will flush right through the system without absorption if the particle size isn't large enough. I never bake my shells (or rinse them) but I do always let them dry out.. not touching each other (or cupped together) in order to not allow mold growth.
Feeding eggs shells does not cause egg eating.. they don't look the same and aren't served in the lay box. Sometimes lack of protein *may* contribute to egg eating and other disturbing behaviors. Too many times though nutrition is blamed for boredom.
I've raised probably a thousand birds.. and had a few happy accidents that got an egg eaten. Keeping wooden or ceramic eggs in the nests helps detour and prevent curiosity pecks from being rewarded.
With all of your birds at the age they will/should molt and not be laying eggs at that time.. I would consider switching to a flock raiser or grower type feed (still with oyster shell free choice on the side).. something with less calcium but more protein and amino acids (feathers are made from 90% protein and it's amino acids). I never use layer feed anymore and since making the switch my birds have softer molts, recover faster and return to lay sooner. It sounds counter intuitive when we're discussing weak egg shells.. but there's more to the story in my experience and opinion than just calcium.
If it was only one of your birds experiencing it I would think personal genetic hiccup.. that doesn't appear to be the case.
What breed birds are we talking about? I didn't see what is your current weather pattern.. if you're in a part of the country that's feelin' the heat right now?
You mention time change.. so you're in the US I figure.. D3 helps with calcium absorption.. I repeat for my own sake that I don't think calcium is the issue (at least not the availability of it) BUT fwiw.. I would consider using a supplement.. like Rooster Booster (brand) Poultry Booster or Poultry Cell (product) to boost the other nutrients knowing that ultimately supporting the immune system via nutrients.. they all interact and support the entire system... and "layer" feed meets minimal guidelines but sometimes more than just the minimum is preferred/required.
Some folks when they are trying to get quick acting calcium into their birds like in the instance of egg binding and it's being used to support muscle action, not to make shell harder is Tums antacids.
I know
@azygous also has a citri cal type supplement, maybe she will have some feedback regarding the whole scenario also?? TIA