Conditions:
~ Run Sourcetree version higher than 3.4.25.
~ Configuration: Tools -> Options -> General -> "File Double Click Action" -> set to "Open File in External Diff / Merge Tool".

In the main Sourcetree panel, in the History view, in the list of committed files, double-click any of the committed files.
~ Expected action: the file is opened in the configured Diff tool.
~ Actual action: the file is opened in the default text editor.

Current workaround:
Run Sourcetree from a version 3.4.25 executable, overriding the latest installed version.