Most relays are sealed in an inert gas so oxidation would be eliminated. It would be possible for one to lose the seal or maybe it never had one. Having looked at the board, iBasso used relays from Japan and I would imagine, good ones.
I have used contact enhancers, starting with Tweak years ago. Tweak would build up and harden, what a horrible item it was. On other, most people use too much as all you need is a thin film and nothing more, even a wipe on and wipe off is better than leaving too much on. I also find that these have a limited life, of a week or so. I prefer to use alcohol and clean the contact points and go from there. I have even used very expensive micro silver enhancer. This to my ear, did nothing and did not improve the signal path. Caig for batteries does show an improvement and works over time but for some reason, in my experience, less so in audio, again, over time. This is based on many years of experimentation, reading and long term use.