Code:
$ pwd
/home/cfillion
$ strace reaper test.RPP 2>&1 | grep Test
stat("/Effects/Test", 0x7fffb62e5750) = -1 ENOENT (No such file or directory)
stat("/home/cfillion/.config/REAPER/Effects/<Project>/Test", 0x7fffb62e1030) = -1 ENOENT (No such file or directory)
$ strace reaper /home/cfillion/test.RPP 2>&1 | grep Test
stat("/home/cfillion/Effects/Test", {st_mode=S_IFREG|0644, st_size=292, ...}) = 0
stat("/home/cfillion/Effects/<Project>/Test", 0x7ffd961273b0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/home/cfillion/Effects/Test", O_RDONLY) = 26
stat("/home/cfillion/.config/REAPER/presets-js-_Project__Test.ini", 0x7ffd9612d510) = -1 ENOENT (No such file or directory)
stat("/home/cfillion/Effects/Test.rpl", 0x7ffd9612d6e0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/home/cfillion/.config/REAPER/presets/js-_Project__Test.ini", O_RDONLY) = -1 ENOENT (No such file or directory)
Also, because the relative path is added as-is to the recent project list, the project cannot be opened if REAPER is started with a different working directory later.