Media becomes unreadable afterwards

I’ve seen a number of comments mentioning that after some period of time, media that was readable becomes unreadable. I’ve been under the impression that when information is written to a CD and DVD becomes permanent. From what I’m reading, this seems not the case. Is this true? If so, is it only some media that has this problem? Is it burner related? Or a combination of both?

its princo media
(bulkcr@p, dat@shite, unbranded)
I’ve had bulkcr@p go away after a week

verbatim and ritek don’t seem to suffer from this.