Your own answer led me to guess at the cause of your problem.
You apparently have an autocmd that automatically saves a view when you quit a buffer, and loads the view again when you re-enter the buffer.
Thus, if you have "options" included in your 'viewoptions'
option, then any options you manually set on a file after the first load gets them from you filetype plugins, will be remembered forever.
Probably you somehow cleared the 'filetype'
option on that one file, so now every time you load the file, an empty filetype is restored from your view file.
Possible solutions:
1. delete the current view file, remove "options" from your 'viewoptions'
setting so it doesn't happen again
2. manually set the filetype, rely on your view file to continue restoring that type forever
Which solution you choose is up to you, and depends on whether you really want all the options you set on a file to be persistent or not.