Hope you are doing well and sorry for the trouble. As described, when applying redaction to some documents, that text below the redacted text just disappears.
Would you mind sharing the version of the Adobe Acrobat DC you are using? To check the version go to Help > About Acrobat and make sure you have the latest version installed. Go to Help > Check for Updates and reboot the computer once and check if that helps.
If the file is stored on a shared network/drive, please download the file to your computer locally and then try to redact and check.
You may also try to repair the installation (Win Only) and check. Go to Help > Repair Installation