The API has traditionally ignored values beyond the allowed limit, returning a warning for this situation since 2008(!). It's long past time for this error situation to actually raise an error, as requested in https://phabricator.wikimedia.org/T41936.

This is happening in https://gerrit.wikimedia.org/r/433742. It should be deployed to Wikimedia wikis with 1.32.0-wmf.6 or later, see https://www.mediawiki.org/wiki/MediaWiki_1.32/Roadmap for the schedule.

Logs indicate that few clients on Wikimedia wikis are hitting the warning. You can check your client by seeing if you're receiving a "Too many values supplied for parameter" warning, or by using Special:ApiFeatureUsage for your client's user agent and looking for a "too-many-X" code.

If your client is affected, the solution is to divide the values into batches of the appropriate size. Generally the limit is 50 values for clients without the apihighlimits right and 500 for clients with that right. The limits for any particular parameter are documented in the auto-generated help and are available in machine-readable format via action=paraminfo.