I’ve been on both sides of it. While not super regular, I’d say it’s not uncommon. Discussing it openly isn’t.
My buddy went to Amsterdam and had the restraint to only get handjobs and has never otherwise gone behind his wife’s back. I’d be surprised if any of my other friends did anything. One couple has guests over for mutual threesomes, so no need for infidelity for them. I know my ex hooked up with another female despite knowing she had a bf. People are ok with lowering their standards when an opportunity presents itself.
I’ve noticed intermittent issues on my instance as well. Perhaps targeted disruption.