Hello Tony,
Thanks for your answer and articles.
Indeed Cloudflare is very good for site performance but it’s need to be well configure to prevent it from doing harm than good.
I switched on the develoment mode and purged all cache but the issue was still there. And has I had another cache issue I desactivated WP-Optimize* cache. After that the cart widget issue no longer occurs. I switched off the development mode and I will wait to see if it happens again.
Regards.
*On staging with WP-Optimize cache actived this issue not occurs. It’s weird.