Uploading New Manifest for Microsoft Teams App gets "no manifest file in your app package" error

Question

Hello! I am currently developing for my app in microsoft teams. When I try to manually upload a manifest file via: https://admin.teams.microsoft.com/ I get the following error:
"There's no manifest file in your app package. Check the app package and try again"

However, there is definitely a manifest file in my zip file.

I noticed the app upload will succeed if I directly download the files from developer portal. However if I unzip and then rezip and try to upload (without altering any contents) I get the above error.

Is anyone able to help me understand why?