That's odd...shouldn't happen unless you really slammed it into something at an awkward angle.

It might just be a dodgy part, so for the time being I'd suggest you just pick up a replacement plastic arm. Only if you keep having issues would aluminium perhaps be an option as more often than not, all but the very best aluminium parts will bend in a bad crash instead of breaking like a cheaply-replaceable plastic arm.
Wow, that's a long and grammatically rubbish sentence lol. Anyway, that's my suggestion.
