I recently ran into this issue and wanted to make a quick note of it.

My YouTube iOS app wasn’t recording my Watch History, but it was working in the browser. I tried all the usual troubleshooting steps of signing out, toggling the setting, delete the app, restarting my phone and nothing worked

I randomly had the idea to check my router’s firewall, I use PfSense and have this plugin called pfBlockerNG installed. I went to firewall > pfBlockerNG > Reports and there is was!- s.youtube.com was being blocked. Adding that URL to the whitelist, I didn’t do a wildcard, and suddenly the iOS starting working again!

I’m not sure what happened as pfBlocker’s DNSBL has been enabled for a while and I had zero issues. My theory is: my Phone’s DNS cache expired, re-fetching tripped the DNS Blocker. This problem seems to happen to all DNS Blockers and is not pfBlocker specific.

This is a very niche problem. I expect anybody using PfSense would most likely be able to figure this out way faster than me. I’m making note of this and adding some specific keywords in hopes that this may potentially help somebody in the future.