That was me.
I had three transmission mounts bolts fall out where I was using stainless steel fasteners and washers.
According to
here, grade 5 steel has a hardness between Rockwell C25-C34.
According to
here, 304 stainless has a max hardness of Rockwell B92.
According to
here, Rockwell B92 has a tensile strength of 96,000 PSI, while Rockwell C34 has a tensile strength of 156,000 psi.
Whether that makes the difference, I don't know. What I know is that when I removed the remaining transmission mount fasteners, all were loose, and the stainless washers were "dished." I do not see the same level of deformation where I've used plated steel washers, so my thought is that the stainless washers yielded over time and allowed the bolt/nut to loosen. I did not use spring washers.
I reinstalled the transmission mounts with grade 8 washers and grade 8 bolts, so we'll see if they fall out again.