I think it's normal, in fact I had a red leg that kept trying a shell out like once a month for almost 6-8 months until he finally stayed put in the new shell. He kept going back and forth. I think the new one was a bit too big but he was set on making it his new home despite the fact that he had a junkyard of shells to choose from.

Wish you and I both had taken pics. Definitely entertaining.