Have you used an SSTV decoding application on an up-to-date version of macOS? Which one, and if you built from source, can you share the steps you took to do so?
I do not know of a single working macOS application for SSTV decoding. I've been playing SSTV signals over my Mac's speakers, captured with my RTL-SDR dongle, and using Robo36 on an Android phone to decode the image, as has everyone else I know, but this comes with additional problems. There have been instances of engine-noise (or even the occasional donkey bray) introducing unwanted artifacts in the decoded image.