@UrbanWild, I have a lot of mosaics that have been outside for years. One is on my husband's astronomical observatory. One is a fish pond mosaic on my deck. I have little lizards crawling around our outside walls. Most have been there for 8 years now. I also have patio tables that are just fine with nothing peeling off. I haven't done many stepping stones.
I use Loctite polyseamseal Clear Caulk to use as an adhesive. I grout normally with sanded grout. I don't seal the work since most seals do not work.
The amount of rain might be the problem there. We only have 10 to 12 inches of rain a year here in NM. The mosaics seem to be ok even in AZ. I have a pool equipment box that has all 4 sides mosaicked, and it's been out there for about 10 years.