NOTE This is both a website bug AND an engine bug.
For both players, Roblox will sometimes randomly show someone you have as a friend as not being one of your friends. In-game, it will not show the friend icon in game and it will show the button to send a Friend Request (but doesn’t work). On the website, it will NOT show them in your friends list and when you check their profile, it will show a button to send a Friend Request.
This bug usually lasts for over an hour, though I haven’t been able to figure out an accurate time. It does resolve itself eventually, though.
This has happened to me with SEVERAL friends of mine and, unfortunately, has caused an argument between me and some of them because it was showing us as not friends but neither of us unfriended each other. Then days later noticed we are actually still friends.
I do not have an example currently, but I was just reminded of it when a friend that was affected joined me moments ago. Once I have it happen again, I will record a video and attach it here. But, it’s still worth reporting because I’ve seen other people that I don’t know discussing it.