disapaering items from palets

"Better select the note you want to apply it to and then double click the dynamic"
In MuseScore 3.4 a single-click will place the dynamic. I am still getting used to this recent change!

I've seen it occasionally too, but so far haven't found reliable steps to reproduce. I believe it's related to timing, though. There is code to decide if the user is trying to drag onto the score or trying to rearrange the palette, and the code involves detecting how long you wait before you start the drag. If you wait more than half a second or so, it assumes you want to rearrange. It still lets you drag, but you'll notice the palette start to rearrange as you drag. It should be harmless as the palette normally resets itself once you drag out of the palette, but I can make the bug happen often - not every time, but maybe around 10-20% of the time - if I do the following:

1) click and hold a palette item in the topmost row or rightmost column of any palette
2) after waiting at least a second, slowly and carefully drag it out of the palette without touching any other cell along the way

You'll notice as you start the drag, the cell will appear empty, because MuseScore thinks you are moving the item within the palette rather than adding it to your score. Normally, once you reach the score, the cell fills itself back in. But sometimes, the cell doesn't fill back in, and when you release the item you are dragging, the cell remains empty.