Ever lose a friend over your weight loss?

I think it's a mistake to say friends who are "real" friends won't leave no matter what. Friendship isn't marriage, they haven't taken a vow "till death do us part." Friendship is a voluntary association, which is what makes it amazing because our friends don't have to be there for us, they choose to be. That said, it's a little presumptuous to expect all our cordial acquaintances to perform at the level of totally committed life-long friends. We all have acquaintances at a range of levels, all of whom we tend to call "friends." All acquaintances becoming friendships build on compatibilities and need-meeting. We are compatible, and we meet certain of each others' needs in some way.
Weight loss programs can disturb both of those. If compatibility is tied in some way to sharing a lot of eating out at rich, exotic eating establishments, "no rules" types of eating, meals that are thought festive by virtue of being excessive, well our weight loss program can surely upset that element of shared enjoyment and compatibility, and put distance into that acquaintance. If the other person recognizes the importance of this and changes to meet us because they value the acquaintance, then you're looking at a real candidate for a closer friendship. But if they choose to let the relationship slip back into the distance, that's their choice, and that's fine. They have that right.
Another way weight loss can disturb the dynamic of acquaintances becoming life-long friends is when the other person is threatened, envious, or jealous, or made to feel guilty or inadequate by our commitment and success. In those cases, their character flaw can either lead to them growing or choosing to retreat, again, it's their choice and we can't judge them for that. They didn't take a vow.
Another way--a very common way--weight loss programs disturb relationships is that our commitment and success makes us arrogant, self-righteous meddlers. We sit in restaurants, use our MFP app to tell others how many calories are in the foods they've already ordered or perhaps are eating. We make this the theme of our interactions with them and become arrogant, self righteous, insufferable bores! Others rightfully recoil from this behavior and decide they don't really need "friends" like that.
We need to learn how to pursue our commitment, and enjoy our success, in ways that don't make us insufferably irritating to others who have made different life choices than us. Where they actively or passively undermine us, we have to make a choice to confront, endure or avoid. But that's our choice, and we should make it graciously and live it out appropriately in our contacts with others.

My friends are supportive, and don't care how fat or skinny I am. Most of my friends are thinner than me, though. So no idea.

I randomly gained friends when I lost the weight.

Real friends won't care if you are fat or skinny. BUT if they are overweight and you are constantly bragging about your new diet, how much weight you've lost, how much better you feel, ad nauseum, you are being cruel to your friend. Yes, it would be great if your friend said, "Wow, he/she is doing so good, I think I will follow his/her example!" and started fluttering all about you, asking for your advice and taking it and losing weight with you.

But that's not usually how it happens. Usually, the friend feels left out because all you want to talk about is your weight loss. JUST STOP THAT and keep your friends!!!!