That may be because the mobile Safari MediaRecorder API only supports the audio/mp4 mimeType. So using the fancy AudioContext / compression API could only do very limited improvements on the low-fidelity output from the MediaRecorder.
Pretty much the only fix is a native plugin or working with the raw PCM using JS libraries which is a big development effort and I get the vibe it’s not worth the cost.
But, thank you to Obsidian team for looking into it