There doesn't appear to be a chain of steps, and appears to be wholly on the backend of vrchat. I've had this in the past with various friends.

It will randomly remove a friend but not actually remove them, they just appear that way. For example, if you send a friend request, it will say you are already friends, but they still aren't on your friends list.

It acts as though you have mutually blocked eachother but for one or both you can add eachother, but will sometimes you'll get a error like you are blocked even though you aren't.

If you look at eachothers block lists, their name isn't on it but it still acts that way. And the only way of fixing it is to block and unblock, then re-add them. It's happened 4 times in the last 2 weeks. And 3 of those times it's been with my own partner, which has led to some serious confusion. We're both habitual pcvr users both on quest's.

Edit: I asked my partner and according to her this bug has existed for at least a month. She had a friend's list over 100 and now it's down to 30 and she doesn't know if she'll ever be able to get them back.