I get the same thing. The only two ways around it that I’ve found is to either copy and paste the link and then continue typing on the next line (because the link is still displayed as regular text until you close and then re-open the cell),

or to type in a bit of text after the spot where you will copy the link. In certain situations, where I know I’m going to be adding more links and text later on in the same cell, I’ve got into the habit of leaving one letter after the link that is regular text.

I think that you shouldn’t have to work this way but that’s the only solution I’ve found.

Actually, this is the way OS X handles text formatting (this is not really about the link, but how it’s formatted). Victor describes a couple of ways of working around this.

You can try the same thing in TextEdit (NoteBook and TextEdit share much of the same Mac OS X text creation and editing code) and see that TextEdit behaves the same way.

OS X is operating on the assumption that at the insertion point, when you continue typing, then you want the text style and formatting to remain the same. This is generally correct. This concept predates links by many years so I agree that links of all stripes have thrown a wrench in the process.