I can't see the leaves clearly under magnification, but check that there's no invaders under the leaves or anywhere else. Otherwise, there's certainly plenty of slug/snail damage...

I agree looks like slug and snail damage. Has it got suitable drainage after all the rain we have had.

It’s been in the same place for years, and this is the fist time this has happened. Would slug/snail damage affect the stems? They are thin and spindly.

A heavy harvest would cause the stems to get thin, and heavy snail damage would keep it from recovering.

I agree with Tugbrethil, over cropping could result in spindly stems

Do you mulch / feed it with manure / compost etc or does it suffer the normal rhubarb conditions of benign neglect?

Once I started adding a good layer of muck over winter mine improved a lot

I think I'd lay off it this year, give it some muck and wait

It’s certainly not over cropping! I’ve only had one small harvest from it this year, but I think “benign neglect” is the answer! I shall feed it and talk to it a lot more!