From my understanding, the actual world generation range is much greater than what's revealed on the map as you're exploring. I want to say it's 500m, but I can't remember if that number is accurate. So essentially there needs to be a significant unexplored area surrounding the mountain in order for the frost caves to generate; if you've revealed the outline of a mountain, the entire mountain has essentially be generated despite not being fully revealed on the map. This only applies to worlds created prior to the frost caves patch.
You can check if any frost caves have generated on your current world on the valheim-map.world site (you'll have to upload your world files to get an accurate, post-patch map). There is also a mod, Upgrade World, that can regenerate biomes for updates like this one and you can specify just the mountain biome in this case. Obviously do this at your own risk and be sure to backup your world files before doing anything in case something goes wrong. I've used it specifically for this reason and haven't experienced any issues.