Problems with highlighting

After updating my MacBook Pro to BigSUR (maybe), I can only highlight one line at a time. I enable highlighting by clicking the marker in the toolbar, but after using it to highlight one line of text, it automatically disables, so I have to click it again for the next line... Very annoying and I've never had this problem before. Usually, I can just keep highlighting untill I press esc or click the highlighter again to disable it, but now it just disables every time. I really hope someone can help me fix this. Thanks in advance.