The log shows that there is an underlying frequency limit of 3 updates per second due to the license in use.
This accounts for the suppression of some update when two or more are produced in short interval.
As discussed above, you can request the item as unfiltered, but you must be sure that the average frequency of updates of this item is steadily less than 3 per second.
Otherwise you can use setRequestedBufferSize and determine a size that is a trade-off between possible lost (i.e filtered out) updates and possible update delays.

This does not explain why an item in COMMAND mode yields fewer updates than an equivalent item in MERGE mode.
We can investigate this case, but please provide some precise references.
If you saw this happening in the run of the provided log, please specify two item names for the COMMAND and MERGE subscriptions with different behaviors.