I expected to see more log, but the snippet clearly shows that a filtering operation was performed by the Server.
The two updates received in fast sequence, with an ADD and a DELETE for the "loading" key, cause the deletion to be performed internally before the ADD event is sent to the client. In this case, nothing is sent at all.

This is the correct behavior, as we consider the key information as obsolete at the time the DELETE is received. Probably, you are not using the COMMAND mode in the correct way; why should you delete the key at that point?
Anyway, you can suppress this kind of filtering by asking the item in "unfiltered" mode, that is, by issuing setRequestedMaxFrequency("unfiltered") on the client page.

There is no reason why the filtering should happen only when your page is loaded in an iFrame. However, this filtering behaviour is exposed to race conditions. The Server might forward the ADD event before receiving the DELETE event depending on the current load and two different page initialization sequences can explain the different outcomes.