From https://bugs.launchpad.net/catfish-search/+bug/1731608 First noticed with an insync google drive folder automatically labelled as firstname.surname@gmail.com. Catfish will not search this folder unless "show hidden files" is selected. To test I created another folder "he.llo", and catfish's behaviour was the same. ubuntu 16.04.3 1.4.2-0ubuntu1 (via catfish maintainers ppa)
Sean Davis referenced this bugreport in commit 8b3fe1149562195b33f73825db21e231caa5aea7 Improve hidden file logic (bug #14497) https://git.xfce.org/apps/catfish/commit?id=8b3fe1149562195b33f73825db21e231caa5aea7
Fixed in the above commit.