I've noticed a bug, whereby if you have already chosen some evolutions/upgrades, then go back to the evolution menu, the upgrades are removed and you have to select/evolve them all over again (and pay for it). It 'attempts' to evolve (ie, you change into an egg) but immediately stops, and you lose your evolutions.
Specifically, I've noticed with the Fade, that sometimes when this happens, it makes you pay for the evolutions AND the 50 res for Fade itself, even if you've already evolved to Fade. However, it only makes you pay for the Fade again if you try to evolve 3 upgrades all at once (ie, carapace, regen, and silence) - the 'res cost' number goes from 2, to 4, to 56 upon selecting the 3rd upgrade.
I hope this isn't too hard to understand - it's happened to me on Lerk and Gorge as well, minus the part about having to pay for the Lifeform again.
EG: I'm a Lerk with Carapace and Regen. Silence becomes available, so I open the menu to select silence. Upon selecting it and trying to evolve, I lose my previous upgrades, and I have to pay for them again.