UE4 is hardcoded to use 755 for files and directories. See mkdir() in Engine/Source/Runtime/Core/Private/Unix/UnixPlatformFile.cpp
This is also repeated for Android, HTML5, and Lumin but i cannot confirm any symptoms on those. In a Linux development environment it is useful for different users to share assets and DDC. 755 forces read only for everyone but the owner, making this difficult. A user should be able to change their umask to override this.
Results: Directories and files were created with permissions rw-r-
Expected: Directories and files should be created with permissions rw-rw-r--
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-79519 in the post.