650 barrel/pushrod length differences

Shouldn't be happening. Might I suggest replacement valves from a reputable supplier?

There is definitely a big hint of soft valve ends here. Proper valve ends were Stellited or stainless.

But.....there are also a few other possible causes to check out.

Post 1965 engines had slightly shorter pushrods to compensate for the loss of the spigot. Later Valve lengths also changed slighly to get the rocker arm angle better. Have you got a mix of components? The adjuster scew on the rocker end usually gives a clue here with either no spare thread showing or too much.

Some people have fitted valve guides with almost zero valve stem clearance. There were quite a few of these on the cheapie bits market a few yeras ago. Some even had the Green Blob Logo hinting at official specification parts. These promptly seize up once the engine gets hot. Check that your valvs have not been tap dancing on the pistons.

Unleaded fuel has not helped with cylinder head refurbishment. Some NOC members have reported cases of the fitting unleaded valve seats and later finding that the heads of standard valves weld themselves to these seats.