Guild Wars 2

For example: Runes of the undead gives 5% condition damage based off of toughness. If I take signet of the stone (180 increase in toughness) my condition damage is unchanged with or without the signet passive on.

This has never worked with passive increases that I know of…be it signets, sigil of momentum, banner of defense, or strength in numbers. This and similar traits, I believe, only work on base stats. Base toughness plus stats from equipment, runesets, and toughness from the trait line. I don’t know if it has something to do with possible power creep or just the wild calculations that would constantly have to be done to check for condition damage with using said runeset combined with other variables I already mentioned.

I can tell you this is consistent across the board though, as all other similar conversion effects are treated the same. For instance, a precision to [insert stat] effect would have to account for +420 precision while fury is in effect. Something like power to [insert stat] would have to account for +875 power with 25 stacks of might active. I hope you see how this could get out of hand from a balance perspective and from a computing perspective.

Tldr: working as intended.