I remember this would happen in certain conditions. Probably if you change the viewport layout or something similar.
Try this:
1. Disable IR
2. Restore all the views (press alt+w if you have one view maximized)
3 When you have all 4 views (or whatever number you are using, but NOT one maximized) right-click on few of them few times
4. Unlock the lock
5. Again right click on multiple viewports
6. Run IR in viewport or floating
7. Select the viewport which you wish to have locked
8. Lock the lock
Does it help?