Is this WWI movie scene realistic?
A loop is about 3G on the initial pull, declining to maybe half a G over the top, then increasing to 3G on the pull out.  If you go in at the proper entry speed etc and you want to make it reasonably round.  If you release enough back pressure to go even slightly negative over the top, whatever is unsecured falls out (as well as all the crap on the floor coming up into your face, and, if carbureted, the engine quitting momentarily).   All it would take is a little push on the stick while rounding the stop, for whatever reason.

Observers weren't strapped in because they had to be free to move around to do their thing. 

So that scene is perfectly plausible.