Turning a morph creature face up is a special action defined in CR 702.36e. It is not a zone transition — the permanent stays on the battlefield the entire time. Since 'enters the battlefield' (ETB) triggers only fire when a permanent moves into the battlefield zone (CR 603.6a), flipping a morph face up does not satisfy that condition.
Think of the face-down creature and the face-up creature as the same continuous object. The card never left the battlefield; it simply changed its visible characteristics. CR 702.36e explicitly states that turning a permanent face up is a special action and does not use the stack, further distinguishing it from any kind of re-entry event.
However, some cards have triggered abilities that specifically say 'when this creature is turned face up' — those will trigger (CR 702.36f). This is a different trigger condition from 'enters the battlefield,' so you must read the card carefully to know which applies.
Concrete example: You cast Stratus Dancer face down as a 2/2 morph. Later you pay its morph cost to turn it face up. Its own 'when this creature is turned face up' trigger fires, but if you control a Panharmonicon (which doubles ETB triggers), it does not interact here — because no ETB event occurred.
In summary: ETB abilities require a zone change (CR 603.6a), and turning a morph face up is explicitly not a zone change (CR 702.36e), so no ETB trigger results.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.