I think boiled egg and chicken is fine for extra protein supplements though
Fresh food just as good as freeze dried, if not better.
It is possible that he might have been getting too much calcium. The science selective contains everything needed (although agree a muesli mix is best unless they are old and struggling to eat). So with another mix as well as the science selective, it's possible he was getting too much calcium. A bit of a long shot maybe.
See what happens when you change hamster mix. I don't know anything about the mix you mentioned from Etsy, but Amity's recommendation is good - and carry on with your own fresh supplements.
It's good to have science selective in though. When hamsters get old, they can struggle to chew hard food and start losing weight, or have other teeth problems. Soaking a few pellets of science selective means they can still eat (soft food with all the nutrients needed). You should always still put their regular mix out as well though because even when they can't eat it properly they still like to have normal behaviours, so they still pouch and hoard it.