The fields do reset the gun, however you can shoot portals in the lift on the way up. There is no such field at the beginning of the next level, so it should remember the last portal shot in the lift.
Between ch 18 and 19, i dont know about grobbo, but my hl2.exe always breaks, and windows feels it stops responding. The window goes 'dead' and the cursor turns into a 'waiting...'symbol for a bit, and then it comes back. This stops Fraps' recording too.
But... I just did 18, and the bug happend, but the portalgun symbol didn't dissapear. I don't know.
But... I just did 18, and the bug happend, but the portalgun symbol didn't dissapear. I don't know.