If you put yourself out there in any capacity--as a teacher, a coven leader, an initiate of anything--someone will eventually come popping up seemingly from nowhere to challenge you on it. It does not matter what your bona fides are; you could be the finest witch with the most impeccable lineage imaginable, and some asshole will still appear to question your validity, deny your right to do what you're doing, tell you your upline is illegitimate, or a dozen other things. And the reason why is simple: people suck.
People suck, and a great many of the suckiest among them can be found in religious and spiritual sectors. Why that is, is a topic for another dissertation, but just believe me when I say that it's so. Among the worst people I have known in my adult life, the bulk of them have been other witches, pagans, and occultists. These people have bullied me publicly and privately, harassed me online, denied the validity of my initiates, claimed I did things I didn't do and didn't do things I did do, tried to ruin my coven, break up my marriage, and the list goes on. I'm far from the only one who has experienced this. A dear friend and initiate of mine once told the tale of attending a public pagan gathering and overhearing some complete strangers discussing him, gleefully recounting all the horrible things he'd supposedly done, with him sitting right at their same table completely unknown to them. If it's true that people love gossip, the more salacious and outrageous the better, then it's magnified a thousandfold in the pagan/Wiccan/occult sectors. It would be hilariously stupid if only it didn't cause real harm.
Why does it happen? One reason is because a lot of heavily damaged people find their way to the craft in hopes of fixing that damage; but as the saying goes, hurt people hurt other people, so instead of finding personal healing they seek scapegoats. Instead of addressing their own feelings of illegitimacy they challenge that of others. Some genuinely deserve compassion; others are genuinely nasty people who deserve only contempt, and our best efforts to avoid them and minimize where possible the damage they seek to do. I've found an inordinate number of that second kind over the years.
As the friend I mentioned above once told me, they behave so badly because the stakes are so small.
I am a part of two Wiccan traditions: the one in which I was originally initiated and trained, and the one which my partner and I developed. I have been questioned and challenged about my place and my rights in both of them at various times, and let me state loudly and clearly that I do not give one single shit whether someone unknown to me, whose opinion means nothing to me, considers me a valid witch or teacher or priestess. And neither, dear reader, should you. The person who starts off with a challenge doesn't want information, they want a fight; they want you on the defensive, want you shaken, want you to doubt yourself. Don't give them what they want. If you're up for a battle, then turn it back on them and question them in turn, but in general it's better to defuse the situation and walk away. Pick your battles carefully or you'll end up with a broken heart and an ulcer, because these people live to drain others dry. Don't open a vein for them.
If someone questions your lineage, tell 'em it's oathbound. Tell them you were brought in by a witch who was brought in by another witch. Tell them you don't have any. Tell them it's none of their godsdamn business. Tell them you made it all up yourself, or you read it in a book, or a wizard in a cave on a high mountaintop passed his wisdom on to you. If you are a member of a tradition and you put yourself out there as such, then you can certainly expect to hear from others of that trad; but there is a big difference between the friendly curiosity of one initiate to another, and the malignant interest of the bully. The sooner you learn to differentiate between the two, and to react appropriately, the better off you'll be. And if you are trying to enter initiate space and need to prove your bona fides, you should already know how to provide a requested vouch. Randos on the internet trying to provoke you or discredit you do not deserve consideration.