I confirm that what you have outlined below works for me too.
But, and this is where we differ, you are highlighting existing text and applying an ID or link to that, whereas I am trying to insert an ID without anything selected.
<p>This is a piece of [text.]</p> - highlighting [text.] clicking Insert ID works as it should.
<p>This is a piece of text.↨</p> (where ↨ represents the cursor) clicking Insert ID generates the error message.
<p>This is a piece of text.↨ </p> clicking Insert ID works as it should.
I think this must be what you said earlier about QT is referencing one character to the right and not the cursor position.
As I said previously, I was more puzzled why it was not working rather than it actually being an annoyance.