There is an error when trying to open another file from Finder when Emacs.app is already running. This erorr is reported in Console:
LaunchedApplication: failed with error -13052 (null){ "ApplicationType"="Foreground", "BundleIdentifierLowerCase"="org.gnu.emacs", "CFBundleExecutablePath"="/Applications/Emacs.app/Contents/MacOS/Emacs", "CFBundleExecutablePathDeviceID"=16777220, "CFBundleExecutablePathINode"=8621172431, "CFBundleIdentifier"="org.gnu.Emacs", "CFBundleName"="Emacs", "CFBundlePackageType"="APPL", "CFBundleSignature"="EMAx", "LSASN"=ASN:0x0-0x49b49b:, "LSBundlePath"="/Applications/Emacs.app", "LSBundlePathDeviceID"=16777220, "LSBundlePathINode"=8621172365, "LSDisplayName"="Emacs", "LSExecutableFormat"="LSExecutable#!Format", "LSLaunchDLabel"="org.gnu.Emacs.5196", "LSLaunchedByLaunchServices"=true, "LSLaunchedWithLaunchD"=true, "LSLaunchEventRecordTime"=154695413870065, "LSLaunchTime"=now-ish 2019/10/24 09:37:28, "LSParentASN"=ASN:0x0-0x2d02d:, "LSWantsToComeForwardAtRegistrationTimeKey"=true, "pid"=50395 }