Spring/Keeper interference with Rocker

Yes... look up 'lifter removal tool.

Check that rocker to see if there are any hairline cracks, including in the pushrod pocket.

Have you removed the pushrods and rolled them to see if they are perfectly straight? And are they the exact same length? Look at the pushrod on that lifter and see if it has any rubs marks on it, and see if it a tad shorter than the rest.

If the head gaskets are Felpro kit gaskets (thicker than stock), and head work was done and the valves 'tipped' (ground on the tips to make them equal length after valve work), and you have that rocker and shaft wear, then you could have enough changes that the wear tolerances could be stacking up and that pushrod could be juuuust a few thousands to short.