Make sure to check that you have the Add members button in the same group in other apps. For example in Telegram Web version — web.telegram.org, as admins of the group could disable this permission for you.
Please comment below if you have any additional information regarding this bug.
Steps to reproduce
As a regular member open a supergroup with the global Add members permission enabled.
Current result
Add members button is missing, however it is present in other apps in the same supergroup. For example using web.telegram.org
Expected result
Add members button is present, as you have this permission.
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