This is due to an OS limitation

If you are encountering this issue, you may be using an iPhone.

Apple allows an app to request "only" 64 notifications to avoid the app creator to spam their users. It's a limit built into apple's iOS. Sadly, there is no workaround. 

I have 2 Octopus Watches paired to my phone and I receive the notifications only for one Octopus Watch

The notifications for your first watch arrive on the "top of the list" so it is below the "64 limitation". Then the notifications for your second watch would like to pop up but Apple blocks it because you are above the 64 notifications limit.