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.
*On staging with WP-Optimize cache actived this issue not occurs. It’s weird.
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