There is a few things which can mess with your DNS - half hearted experiments with
- VPN
- The plethora of privacy guides which instructs you to change your dns settings
- Some even suggests making resolv.conf immutable.
- Adblocking on DNS level e.g. hosts manipulation
- Pihole
- A change here - a change there - and no record of what, where and when - making it next to impossible to roll back
A basic troubleshooting guide is available