Steps to replicate the issue (include links if applicable):

• Open https://en.wikipedia.beta.wmcloud.org/wiki/Help:MathTest with your current math rendering settings
• Add a mathmode= parameter to the URL whose value is not your current math rendering setting and reload the page

What happens?:

• The mathmode parameter is ignored and the page is rendered with the previous setting
• The cache key that is accessed (as indicated by the limit report) is the one corresponding to the user setting
• Purging the page before rendering with a custom parameter inserts the wrong content in the user setting-keyed cache entry

What should have happened instead?:

• Requests with mathmode= should access the corresponding cache key rather than the user cache key