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One crazy-ass thing about atomic nuclei is that they actually have less mass than the sum of their parts. The more stable the atomic nucleus, the more mass is "missing" relative to the isolated baryons.

https://www.nuclear-power.com/nuclear-power/nuclear-energy/mass-defect/

So as nuclei are marching towards the stable ones at the end of their decay chain, they lose mass not simply because alpha and beta decay evolve ejecting particles with rest-mass, but also because more mass disappears into the nuclear binding energy.

I agree it is weird, but it's observable, not theoretical.

In short, it is no longer considered a principle rule of the universe the matter can not be created or destroyed. Rest-mass can convert to energy (indeed, at small scales, it happens all the time, like in the banana on your counter).