Highlighting no longer working in Preview

The Highlight Annotation remembers the last Highlight selection, so if you last chose strike through text, that is what you will get.

I suggest that you do not use the Tools > Annotate sub-menu for HighLight Annotation, but rather, expand the width of the Preview window until you see the Highlight tool. Press the command key as you click on it, and then drag it to the left until it is in the following position. This will make it easier for access, and color selection.

Then reduce the width of the Preview window until you still see the Highlight Annotation tool on the toolbar.

You can right-click over an annotation, including that strike through to remove it.

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

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, wasn't aware of that! :)

You are welcome.

FYI: For older macOS versions, you may need to View/Show Toolbar and Show Markup Toolbar.