Same result in python (using requests_oauthlib):
>>> from requests_oauthlib import OAuth1Session
>>> twitter = OAuth1Session('redacted', client_secret='redacted', resource_owner_key='redacted', resource_owner_secret='redacted')
# With '@':
>>> url = 'https://api.twitter.com/1.1/search/tweets.json?q=@Morten242'
>>> r = twitter.get(url)
>>> r.text
'{"errors":[{"code":32,"message":"Could not authenticate you."}]}'`
# and without '@':
>>> url = 'https://api.twitter.com/1.1/search/tweets.json?q=Morten242'
>>> r = twitter.get(url)
>>> r.text
'{"statuses":[],"search_metadata":{"completed_in":0.005,"max_id":875252928604274688,"max_id_str":"875252928604274688","query":"Morten242","refresh_url":"?since_id=875252928604274688&q=Morten242&include_entities=1","count":15,"since_id":0,"since_id_str":"0"}}'