Don't use state converter on invalid JSON and the payload is not on, off or toggle. https://github.com/Koenkk/zigbee2mqtt/issues/1348

lib/extension/publish.js

@@ -112,8 +112,11 @@ class EntityPublish extends Extension {
             try {
                 json = JSON.parse(message);
             } catch (e) {
-                // Cannot be parsed to JSON, assume state message.
-                json = {state: message};
+                if (['on', 'off', 'toggle'].includes(message.toLowerCase())) {
+                    json = {state: message};
+                } else {
+                    logger.error(`Invalid JSON '${message}', skipping...`);
+                }
             }
         }
 

test/publish.test.js

@@ -559,6 +559,14 @@ describe('Publish', () => {
         expect(MQTT.publish.mock.calls[1]).toEqual(["zigbee2mqtt/wall_switch_double", stringify({state_left: 'OFF'}), {"qos": 0, "retain": false}, expect.any(Function)]);
     });
 
+    it('Should not use state converter on non-json message when value is not on/off/toggle', async () => {
+        const device = zigbeeHerdsman.devices.QBKG03LM;
+        const endpoint = device.getEndpoint(2);
+        await MQTT.events.message('zigbee2mqtt/wall_switch_double/left/set', 'ON_RANDOM');
+        await flushPromises();
+        expect(endpoint.command).toHaveBeenCalledTimes(0);
+    });
+
     it('Should parse set with postfix topic and attribute', async () => {
         const device = zigbeeHerdsman.devices.QBKG03LM;
         const endpoint = device.getEndpoint(2);