Set Mode 2D/3D Errors

I’m playing back sound through FMOD which I want to switch from 2D/3D mode on the fly. To do that I’ve got some code like this:

// Nb. `_channel` is an `FMOD.Channel`

if (positional)

    Check(_channel.setMode(MODE._3D | (MODE)RolloffMode), "Failed `setMode`", true);
    Check(_channel.set3DMinMaxDistance(MinDistance, MaxDistance), "Failed `set3DMinMaxDistance`", true);
    Check(_channel.setMode(MODE._2D), "Failed `setMode`", true); // <-- ERROR HERE!

However, when I run this I’m getting errors on the final line:

Failed setMode. FMOD Result: ERR_NEEDS3D

This appears to be telling me that I simply cannot set this channel into 2D mode! What am I doing wrong here?

Edit: I’m calling this every frame. So is the problem that I’m calling _channel.setMode(MODE._2D) on a channel which is already 2D?

You should be able to set the mode of a channel at runtime, what version of FMOD are you using?
Are you playing these channels using the Core API, via System.createSound, or the Studio API, creating events and getting their channels using EventInstance.getChannelGroup? If you are using the Studio API you can try toggling the byppass state of the spatializer (“FMOD Pan” dsp):

void Toggle2D3D(FMOD.Studio.EventInstance eventInstance)
    eventInstance.getChannelGroup(out FMOD.ChannelGroup cg);
    cg.getNumDSPs(out int numDSPs);

    // Linear search for spatializer
    for(int i = 0; i < numDSPs; ++i)
        cg.getDSP(i, out FMOD.DSP dsp);
        dsp.getInfo(out string dspName, out uint version, out int channels,
            out int configwidth, out int configheight);

        // "FMOD Pan" is the Spatializer DSP setup in Studio
        if (dspName.Contains("FMOD Pan"))
            dsp.getBypass(out bool bypassed);