Enantiomerically pure ketone reaction under acidic conditions gives 1:1 mixture of diastereoisomers?
Carbonyls are able to undergo keto-enol tautomerisation, this is accelerated in the presence of acid/base. 

This phenomenon means that alpha-chiral carbonyls are generally regarded as configurationally unstable, as they are prone to racemisation, although in the absence of acid/base the extent to which the alpha-chiral carbonyl exists in its enol form is often negligible (the C=O, C-H combination is ever so slightly more favoured than C=C, O-H in a lot of cases).  

The enol form of the carbonyl (the middle structure in my diagram below) is planar. When the enol tautomerises back to the ketone form, the proton may be transferred to either face of the planar structure, leading to both diastereomers being formed, as you describe.