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Our Canon R5 Resources: Memory Card Tests | Accessories | Firmware | User Guide | Price Check: B&H Photo
Although this does not directly address your question it is worth remembering. On the odd occasion when I thought the lens was not auto focussing and freezing I had accidently moved the AF point by touching the back screen with my nose or a finger. The lens was acuually focussing but way to the side if my subject where I was not looking.Jake brings up good points.
One big thing that changes when you zoom in to 500mm, is that your minimum focus distance increases, so if you're trying to focus on an object that's too close, you might be able to achieve focus at 200mm, but not 500mm.
If this problem exists when you're trying to focus on infinity, a distant object, then it's likely that your lens is faulty.
Our Canon R5 Resources: Memory Card Tests | Accessories | Firmware | User Guide | Price Check: B&H Photo