This is the expected behavior at the moment. It may change in future.
The “View as Message” selection is not persistent. When Topics is enabled and View as Message is enabled , after you navigate out of the particular channel … and come back , View gets automatically reverted to ‘View as Topic’ by default.
Problem : The “View as Message” selection is not persistent - some people who are part of the channel prefer permanent message view even after Topics is enabled.
iPhone 12 Pro , iOS 16.3.1 , Telegram iOS 9.4