Doesn't detect makefile (all lower case) as a Make project
Loading a project via the "Select a Folder..." option or via "Recent Projects", if the existing project uses all lower case name for makefile, the project's build system is set to "Default" and not the required "Make"
Workaround: Create a empty Makefile (Capitalised) file.
From https://www.gnu.org/software/make/manual/html_node/Makefile-Names.html
By default, when make looks for the makefile, it tries the following names, in order: GNUmakefile, makefile and Makefile. ... The first name checked, GNUmakefile, is not recommended for most makefiles
Gnome-builder: 3.32.2