The nuclei Rb83 and Rb85 have the same charge, but Rb85 is heavier than Rb83. While gravity acts more strongly on Rb85, this is probably not the factor producing the stability of Rb85. So, why does the orbital electron fall into the nucleus of Rb83, and Rb85 is stable?
It is not a matter of "falling in": all s orbitals have non-trivial probability densities at the center.
It is about energy balance in the nucleus.
Kr-83 is a lower energy configuration than Rb-83 by enough to make up for the neutrino and the gamma(s). Evidently Kr-85 is not a sufficiently lower energy state than Rb-85.
If it's useful to think of an electron as something that's in a place, then RB83 shows that it can fall into the nucleus. That can happen.
What is it that gives RB83 a half-life of 86 days? If the rate-limiting step is the electron falling into the nucleus, then electrons don't fall into RB83 nuclei very often.
Or maybe something else is the limiting factor. Maybe electrons fall into RB83 nuclei all the time but the nuclei usually don't let them stay.
It falls in. The nucleus spits it out. Over and over and over again. But in rare cases RB83 doesn't spit it back out, and RB85 always does.
That's a possible way to think of it.