Seems many users reporting the same problem on S9 and Samsung Phones. Reasearching same issue NOW using S9+ with Rode Wireless Go. Where the mic IS recognised in Native camera App BUT NOT recognised as external Mic in 'Open Camera' OR 'Filmic Pro'.
Did you find a solution for use with Filmic Pro or Open Camera
Does an external Mic work in Samsungs Native Camera App?
Highlights problem with S9 (no solution.. Comments say Saumsng was NOT helpful in ANYWAY to provide a solution - citing issues with 3rd party Mics, when the problem 'appears' to be related to Samungs Implementation of the Android operating system)
I found this comment also
Yes I can confirm that for it to work CONSISTENTLY you MUST plug in the adapter first (without the microphone) and then Plug the microphone into the adapter.. And you MUST do that EVERY TIME you unplug and plug it again. That is the ONLY WAY this will always work.