Interesting! There is a limitation regarding active text boxes. But this should bother you only if you have a clip in your clipboard.
It looks like that when you work with text boxes ⌘+V is also treated as OS paste command. So what happens is that:
You press ⌘+V, Excalibur executes Paste command on Premiere Pro level. Also OS executes Paste command as well. Thus you get double paste.
I made a simple test and reassigned Premiere paste command to anything, but ⌘+V (I don't have it assigned to Excalibur paste either). After that when I have text box activated, I tried both new Premiere paste command and ⌘+V. Text was pasted both times.
I guess what you should relearn in Premiere if you want to continue to use your setup, is to use ⌃+V for pasting text.
