How can I turn off the 'highlight text' function in Preview?

It's in the toolbar. If you notice, when you do the keystroke, the highlight button in the toolbar becomes "highlighted". You can click it again to turn it off.

I see this sometimes, and I never got to ascertain what triggers it.

To unhilight, I select the highlighted text again.

Then, with no text selected, one can press Command-Control-U and the Underline becomes active instead (or Command-Control-S for strikethrough).

It can even be handy, for example if you are about to underline a series of words or sentences, since you don't need to press any key or select from the menu; but of course you - and I - would like to know how to turn this on and off. If you discover, please let us know.

I have just tested this again, and now I know how to turn it on... but still don't know how to turn it off.

If one presses Command-Control-H (or, respectively -U, -S) when there is NO text selected, it turns the respective tool on; then subsequently, any selection you make will get that annotation applied (highlight, underline, or strikethrough).

I tried pressing the shortcut again, pressing it with also the Option key, and going to the Annotate menu and none of that turns this off...

I've not encountered that, which OS version are you running?

Start with this...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes or more.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

Thanks! I would not have guessed that.