Hope you are doing well and sorry for the trouble. As described, highlighting with different colors with 30% opacity changes the font color as well.
I have tried to reproduce the issue on my end (Win OS with latest version of Acrobat) and its working fine and not changing the font color. Would you mind sharing the version of Adobe Acrobat you are using. To check the version go to Help > About Acrobat and make sure you have the application updated to the latest version Go to Help > Check for updates.
Are you on Mac or Windows OS and what is the version of the OS?
Please try to repair the installation (Windows Only) and see if that works for you. Go to Help > Repair Installation and reboot the computer once.