The titles parameter is broken on Special:ApiSandbox.

Reproduction steps:

1. Go to https://en.wikipedia.org/wiki/Special:ApiSandbox
2. Under action, select query
3. Click on action=query
4. Under prop, select revisions
5. Under titles, enter Wikipedia:Village pump (technical)
6. Click on "Make request"

As can be seen in the results, the titles property is there, but it is always an empty string.