I had tons of issues with these Cloudflare checkboxes. I finally figured out it was because I use this extension [1] that disables HTML5 autoplay. I assume Cloudflare is doing some kind of thing where they verify that the client can playback media, as they assume that headless browsers or crawlers won't have that capability
[1] https://addons.mozilla.org/en-US/firefox/addon/disable-autop...