In the security page on Google's account site there is a card with the title "Your connections to third-party apps & services". Go there, click "See all connections" and click Reddit. From here you can remove access for Reddit and/or delete all connections between your Google account and Reddit.

This is actually a google account feature called “auto signin”. If you go to google password manager and click on the settings button in the top right there should be an option called Auto Sign-in you can disable.

If you're on Firefox, take a look at the Containers extension. It helps keeping everything separated

myaccount.google.com

Go here.

Go to the left side where it says Data & Privacy.

Scroll down to Data from apps and services you use

Under that is Third-party apps & services

Scroll down to Reddit

Click Delete all connections you have with Reddit

this should be impossible unless you give it access to use your google credentials. you should be able to turn it off in your google security settings.

This is why on the occasion a thread crosses a reddit post it gets opened incognito.

https://libredirect.github.io

Doesn’t answer your question but might setup this on your work computer.

Make sure your login credentials aren't saved in the browser and keep logged out of your Google account elsewhere. It would be easy to have all your cookies deleted when you close your browser.

Google has a function to limit or remove third party connections/accounts from your google account. You should be able to change that if you wanted to.