Unfortunately i wasnt recording when the issue occured, and didn't realise exactly what had happened until a little bit later. I *think* that i picked up the powerup and almost immediately died by falling into a hole that i had dug next to it, but i can't be 100% sure. Once it changed to the next level however everything went back to normal, although it did persist through death on the same level.
I have not been able to replicate it again since, but will keep a look out and let you know.
Thanks for the tip on screen 16, i didn't realise you could go onto the exit square!