A new issue as of Moodle 3.8 this is the cause of an overzealous Filter on your selection of students. It can be toggled from the View all submissions page.
In most cases this will be due to insufficient space or too small a screen.
Sometimes a student submission may appear totally blank or may appear unchanged from the file you provided them. This is a quirk of PDF editors and files and here are some things you and your students can do to smooth out the grading process.
Sometimes a PDF file isn’t saved right or the conversion process will add a blank cover page.
Check to see if there’s more than one page present from the page navigation controls to the top left of the previewed document, your grading may actually start on page 2.
Chances are that your student used a quick and convenient way to save their PDF that doesn’t play nice with all other PDF readers, including Moodle. This is most often seen from students using a Mac and the default software of Preview (as well as a number of other applications.)
NOTE: this will often add a blank cover page to the exported file.
This can be easily remedied by having students ensure their file is properly exported: