The only scenario that I can recall is if the json data in the COMMAND item is much heavier than the field-based data in the MERGE item and the client is very slow at handling the updates.

Apart from that, we should follow the updates trip to see what happens.
We can start from the Server side with the following settings in lightstreamer_log_conf.xml
- LightstreamerLogger.subscriptions at DEBUG
- LightstreamerLogger.subscriptions.upd at DEBUG
- LightstreamerLogger.pump at DEBUG

Note that the log is verbose, but we assume that you can reproduce the issue with only the two mentioned clients connected.
On the other hand, if you can replicate the issue only in a production scenario with many clients connected, please clarify that first.