Fitted some basin taps for a customer, couldn't get a tap connector flexible or otherwise to seal on the hot. Nut would do up tight but the washer wasn't meeting the seat & the flexible could turn.
He'd bought them from our local 'proper' plumbers merchant not one of the sheds. I returned it & the hot tap in the new set was the same, but the cold was again fine.
Ended up using the body of the cold & the bits from the hot to build a new tap.
Looked at the thread with a magnifying glass, seemed perfect.
Could it have been thicker plating on the hot?