What you are doing is "cross chaning", that is using the small chainring with the smallest cogs. It's hard on the chain and there is no reason to do so. The 50T chainring with a 19T cog will give the same gear as a 34x13.

Since the chain is angled so severly in those combinations, I expect the chain is rubbing on and catching on the ramps and other shifting enhansements on the 50T chainring and that is what you are experiencing.