This usually happens because you have changed the world generation name but not told the server what world to load. To resolve this just ensure that the following match in your server config file
If you have the file name correctly as shown above and it's still resetting, make sure the line does not begin with # (see example below). If there is a # at the beginning of the line remove the # and save the file.
A # at the start of the line indicates that line is commented out in the file, therefore the setting is inactive.
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