Is this an issue with a particular PDF file or with all the PDFs? Please try with a different PDF file and check. If the file is stored on a shared network/drive, please download it to your computer first and check again. Also please share the file with us for testing.
What is the workflow/steps you are doing to sign the PDF? A small video recording of the same will be very helpful.
Also, make sure you have the recent version of Acrobat DC(22.1.20169) installed. Go to Help. Check for updates and reboot the computer once.