There is nothing wrong with the deployment URL getting changed – it is common for Google to perform this redirection.
The issue is can be rather a permission issue. Make sure you deploy the WebApp as “Anyone, even anonymous”.
However, currently I am experiencing the same behavior like you due to a multiply reported recent bug:
https://issuetracker.google.com/72798634 https://issuetracker.google.com/165350842 https://issuetracker.google.com/166010550 https://issuetracker.google.com/166320373 https://issuetracker.google.com/167692852 https://issuetracker.google.com/169349069
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