I did right-click > Close All , and ever since that I've only been able to keep 1 tab open. If I click on a different file, it closes the previous file. I have the setting Workbench › Editor: Show Tabs enabled, but it still doesn't let me open additional tabs. Only if I double click the file it opens it in a new tab, but I would like this to happen with a single click. I've tried restarting VSCode, but the problem persists.
I managed to solve this by disabling the setting Workbench › Editor: Enable Preview.