When you are both logged in to the ACP and frontend with your admin account and log out from the frontend then XenForo automatically logs you out from the ACP as well. This is very bothersome because this way you have to log in to the ACP with another browser if you want to test another non-admin account. So please make the ACP and frontend logins independent from each other.
If I'm in the Admin CP and logged into the front end, but want to test the front end as a different account, I simply Right Click my board link in the top right corner of the Admin CP, and select Open link in incognito window (Chrome, but all browsers should have an equivalent).
I'm then free to log in with any account I like to do my testing and they remain independent from each other.
It's similar to your workaround, but I find this pretty seamless.