• Cisco was started by a husband & wife duo - Sandy Lerner and Lenard Bosack
• Steve Jobs did not start Pixar - depending how far you go back, it was Alexander Schure that gave birth to the original roots of the company (as CGL) and later on, branched off by Edwin Catmull and Alvy Ray Smith.
• Elon Musk did not cofound PayPal - He founded X.com and that company merged with Paypal which was founded by Peter Thiel, Max Levchin, Luke Nosek and Ken Howery)
• Same Elon did not cofound Tesla - it was started and incorporated by Martin Eberhard and Marc Tarpenning
• Etsy was not cofounded by Jared Tarbell - It was a trio of Robert Kalin, Chris Macquire and Haim Schoppik that launched the initial company.
I could go on but my memory is escaping me at the moment. I am sure other people have more examples.
tldr; this is common enough. :)
I have seen some startups even use this technique themselves to "erase" past or exited co-founders. Once you edit the AngelList, Crunchbase, team page, etc, you really can erase the existence of co-founder or early team member(s) if desired because eventually no one will look hard enough to find the early material.
It's something that bothers me a lot actually. I'm working on a project now to track founders and early teams across startups to get a better understanding of how often it happens and if there are any discernible patterns.