Is there a red square with a red plus sign within it on the lower right corner of the text frame? If so, making sure that the Type>Show Hidden Characters option has been enabled, try pulling the text frame open and see if either the type reappears or there are any blue symbols showing. If you see anything other than a hashtag (#) symbol especially a blue "v" shape try deleting it and see if the missing text reappears.
I´ts just an empty blank space, all the other pages have their page number correctly. I attach two screenshots, Page 3 is where you can see the page numbers correctly and page 4 is where its just gone. Not sure if its a help to undersstand the issue better.