The valve is NOT de-laminating from bladder! It's leaking in the valve, between the nipple and the hole.
No idea what it's called.
Just not a tight fit. Seems like it anyway.
Will let you know if it happens again. No wind lately.
Well it's good news then and easy to fix. Lots of info on the web on how to enlarge the valve cap by dipping in boiling water, forcing up the cap a round object (I use drill bits of appropriate size) so the rubber expands, then dipping in ice water and the cap retains the larger diameter. Voila tight fit.