I had a similar issue when upgrading from v0.19 to v6.0.0. I have an editor for JSON in a SvelteKit+Vite project. This did throw the following error:
CodeMirror plugin crashed: TypeError: tags3 is undefined
Clearing node_modules didn’t solve anything, and there are no v0.19 packages left in my dependencies. The workaround by Francesco (May 5) works this for me, wrapping highlighter.style with a function that passes tags || [] to the original function.