I find that winter or late last season vole damage contributes to late sprouting in the spring. Do you have voles?

To my knowlege I've never seen a vole here or anywhere else for that matter.

I rarely see them, just their damage . . .

If it is vole damage, you will have shallow tunnels that cave in with hand pressure in the soil around the clematis. They like clematis roots.

Winter damage is usually tunnels at the interface between the snow and the soil which leave plants chewed to the soil if it's frozen. If it isn't frozen it often looks almost plowed over large areas.

I don't seem to have anything like you're describing-no soft tunnels to collapse, ground is not disturbed around the plant.
Just a mystery I guess.