Quite a few people use cases for a variety of mods. I've seen a lot of aliens with silicone cases, both the cheap chinese ones (like I've got) and the nicer thicker textured ones.
Only issue with the sleeves I've found, the seam is kind of sharp where they molded them together. I scraped it with a razor blade and heated up the silicone with a mini torch and smoothed it out (not while it was on the mod). That and if any juice happens to leak around the atomizer it seems to get caught between the mod and inside of the sleeve. Makes a bigger mess and the mod has to be wiped off and the sleeve wiped or rinsed out and dried.
Some people haven't had the best of luck with aliens but it's pretty much any mod. Quality control isn't the best. I've seen people post about their wismec's having issues, their sigelei's, problems with hohm slice/wrecker mods, even dna mods. Just in the past week or so I've heard two or three people with the lost vape paranormal that had mods that died in a week or less. When it comes to things like the 510 pulling out of the mod, I've seen it with wismec, smok, aleader's funky 160 mods and a number of others.
Likely just luck of the draw and a flawed mod, highly doubt the case caused it. The only venting on the alien is up top with 6 pin holes on either side of the fire button and all the sleeve's I've seen have slots cut out to match. Only time my pcb got pretty hot on mine I left it sitting outside while working and the sun started to beat down on it (started in the shade).