Acrobat doesn't allow to save the PDF after applying a watermark

This is a problem I've been experiencing for a while. I recently changed my computer, but the issue persists, so I believe it's a bug:

Every time I open a PDF document and apply a watermark (either by creating it from scratch or applying my own preset), the save button is disabled (grayed out). If I close the document and reopen it, the watermark is not there. The only way I've been able to resolve this is by making a random change (like moving an image and puting back where it was), and only then the save button becomes active again. It's really annoying; I don't understand why this is happening. I hope I'm not the only one!