I'm pretty sure this is a bug.
Steps to reproduce:
1 - Create new midi item. Make sure item source is not looped.
2 - Create pooled copy of midi item.
3 - Drag any of the pooled copies to the right to (try to) extend the item.
Note that the item source is not looped, and yet I am getting "loop points" (circled), and I cannot add anything to the right of the loop points in the midi editor. To clear the loop points, I need to unpool the item and then drag the item any amount to resize it. As soon as the unpooled item is resized any amount, the loop points disappear and normal midi editing can resume.
Reaper 6.19, Win 10 64 bit