I have Pocketbook Ultra and the same thing is happening to me. What I know:
- It only happens in the default Adobe Reader, not in FBReader.
- I'm currently using firmware version 5.14, which is the latest for my model. It might have been present in the previous one.
- A few years ago it was not happening, but I am not sure when it started and whether it is connected to any firmware updates.
- Not all links trigger this behaviour. Those from Push to Kindle are what I encounter most often, so I can't rule out the possibility that it's caused by something the plugin does. I think I've seen it happen with other links (in regular ebooks), but I'm not sure and I don't have an example at hand. Some links don't trigger it, though.