Can't help with glues, but first I would try with some double sticky tape, perhaps. Also, for future reference, since the facial interface is detachable, it seems they can issue spare parts (there's a thread about that), and I think @vrcover has a kickstarter about multiple after-market types (https://vrcover.com/blog/). More flavours, more choice.
It doesn't look like you will damage anything on the actual HMD and it looks like the pressure of the HMD on your face will hold it in place. If this is the case, I wouldn't bother spending the time and effort to replace it from oculus unless the part is easily obtainable like an online store with the part etc. I also don't think any third party manufacturers are making any now. And no, it hasn't happened to me or even read anybody that this has happened to so far!
I would contact Oculus. Who know, maybe it wasn't glued properly and they will send you a free replacement part under warranty. I don't think this part break so easily.