Patrick Holzer wrote:

I am assuming this is common and due to moisture, surface flexing,
heat, etc. The question is: Is there anything I can do to keep this
from happening?

no, its not common, shower tile gets a lot more wet

just chip it up best you can and maybe get a shop vac or small wisk
broom

you can buy little cartons of grout...5 and 6 dollars...and just mix a
little at the time
in a small butter container...you don't even need a spoon..mix it thick
and then
use your fingers...pack it in...go across it with a damp sponge after a
few mins
let it dry a little

you could always set a toaster up there and block it