So, I was having what you could describe as a weird ass problem. Basically, I'd get DNS errors (CORS request did not succeed), often resulting in broken CSS and cryptic browser problems such as tabs that refused to load.
It only happened when using both browser (firefox) DoH and Split Tunneling Include Mode, with the browser as one of the inclusions.
After hours trying to understand what the problem was exactly, I finally figured it out: it was ipv6. To be more specific, it was Firefox's ipv6 setting. Disabling it fixed the issue completely.
Source: r/ProtonVPN · by /u/brainrot_award