There are two reasons which may be causing you to lose your manager permissions:
Reason 1: Adding learners while still being logged into your account
If you are logged into your account while adding new learners, your account will be placed in the learner group intended for the learner and your manager permissions will be removed. To prevent this, log out of your account first before adding new learners. Once you are logged out, you will be able to add new users to the workspace without losing your manager permissions or being placed in another learner group.
Alternatively, you may use incognito/ a new private window while still being logged in on your normal browser while adding new users. By doing so, your account will not be moved and your manager permissions will not be removed.
Reason 2: Using the registration link to log in after successfully creating your account
When the registration link (i.e. workspacename.lobsterink.com/learn/join or an email invitation) is used to log in, the platform resets your account and your manager permissions are removed. This link should only used when you're signing up for the first time.
As you have already successfully signed up, the /learn URL of your workspace (workspacename.lobsterink.com/learn) should be used going forward when trying to log in to prevent your account from being reset and having your manager permissions removed.