I have a user that regularly uses Adobe Acrobat DC, and he's got a persistent issue (now on 2 computers) with this square comment tool disappearing, the actual tool turns invisible when you mouse over it, then when you select it, your cursor disappears, and any squares you make turn invisible after being created.
I've logged into this computer on my account, and haven't had the same issues- so I assume that it's local to the profile.
Uninstalled/reinstalled Adobe DC Perforned a repair installation Uninstalled, deleted all local (C:\Users\USER\AppData\Local\Adobe and C:\Users\USER\AppData\Roaming\Adobe) adobe files, then reinstalled. I've reset the tool bar a dozen times.
I'm running out of ideas. The tool even disappears when creating a custom comment toolbar, and mousing over the square tool in there.
Hope you are doing well and sorry for the trouble. As described the square comment tool is disappearing.
Would you mind sharing the version of the Adobe Acrobat DC you are using? To check the version of the application go to Help > About Acrobat and make sure you have the latest version 21.01.20142 installed. Go to Help > Check for updates and reboot the computer once.
If it still doesn't work, Please try to repair the installation (Windows Only) and see if that work. Go to Help > Repair Installation.