Line 100: entry X, I add a new target term at the first position
Line 150 already contained a newer source term than the one in Line 100.
I had to reload the glossary in order to make the entry in Line 100 visible again.
Here's another glitch. When a glossary is set to Read-only, you can open a term pair in the Term editor. When you add another target term and close the dialogue box, the glossary pane is updated and the new target term (alternative) is displayed.
It just looks like if the Term editor overrides the Read-only setting (which would be a good idea, since calling this dialogue box can only be a deliberate action).
However, the new target term isn't really added to the glossary file.