We just upgraded our CiviCRM sites to 5.4.0 and found out that Search | Activity Search always leads to an empty result set. We're on Joomla 3.8.11 but I see the same result on the demo civicrm sandbox (which is running 5.6 alpha 1 on Drupal). Activities are still in place - they show up on contact pages and in the search builder. Is there a change in functionality on the Activity Search page that we're not aware of or is this a bug? Anyone able to confirm?
I could replicate this issue on my local machine, this seems to be a core bug and have logged issue at https://lab.civicrm.org/dev/core/issues/353.
So in the mean time until the bug gets fixed you can rely on this option (hope it shoes correct count of results ;))
I have also submitted PR for this at https://github.com/civicrm/civicrm-core/pull/12713 if you can please try after applying patch and then comment on PR about your QA that would help to speedup the process of merging the code in next release.