If I play Cruel Patron (purple MF) to a battlefield where I sacrifice a unit to pay its additional cost, does the hidden card on that battlefield get cleared?

Ruling: The hidden card stays on the battlefield. Hidden cards are only removed during cleanup if there is no unit controlled by the same player present on that battlefield, and Cruel Patron is present when cleanup occurs.

Sequence:

• You play Cruel Patron to a battlefield and sacrifice a unit there to pay the additional cost
• Cruel Patron resolves and enters the battlefield
• A non-combat showdown starts because control was lost when the unit was sacrificed
• Cleanup occurs after Cruel Patron has resolved
• Since you now have a unit (Cruel Patron) on the battlefield, the hidden card is not removed

Nuances:

• The hidden card removal rule (523) checks for presence of a unit controlled by the same player, not control of the battlefield itself
• Control of the battlefield is established at the end of the showdown
• Hidden card rules are expected to receive updates in future versions due to unintended interactions