That seems to be a very strange glitch, I have no idea what's causing it because to me it doesn't happen. I saw it happen on someone's stream earlier so it's concerning. (Was that your stream?) It doesn't happen to most players either, it seems to be a very rare occurrence but it's been reported once or twice before. Lives aren't supposed to reset unless you lose them all, there are yellow Sun Crystals to collect and gain more. I'm not sure what you mean with the character select.