I am nor 100% sure but I suspect this is related to how iCloud behaves. Especially on the iPad, the OS decides what you need to keep on the iPad and what should be downloaded on demand from iCloud. The theory is good but in practice it does not work for me.
I have numerous posts on this forum on how unreliable iCloud is , the latest thread is here. In order for Obsidian to work for me on my Macs and Ipads, I have to pay for Obsidian sync. That is the only way I can make it work.
Not sure this helps you