Mysql row-based binary logging behaving different in some versions of mysql (rows grouping)
The setting is a &quot;soft&quot; max.  If a row exceeds the given value, the entry will be bigger than the max.  That is, &quot;no problem&quot;.
If your desire is to avoid them ever being batched, you probably need the value to be less than 8K.
From the 8.0.14 Changelog:

The startup option --binlog-row-event-max-size now has a corresponding system variable binlog_row_event_max_size. The startup option and system variable set a soft limit on the maximum size of a row-based binary log event, with a default setting of 8192 bytes. Where possible, rows stored in the binary log are grouped into events with a size not exceeding the value of this setting. If an event cannot be split, the maximum size can be exceeded.
The binlog_row_event_max_size global system variable is read-only and can be set only at server startup.

As for whether, or when, Amazon will provide a way to set it, I don't know.  File a bug report with them.
I would suggest that the value of the variable is only a minor optimization.
----- 2017-04-10 8.0.1 Development Milestone &amp; 2017-04-10 5.7.18 General Availability -- Bugs Fixed -- Replication -----
Binlog_sender, which writes events from the binary log to a packet buffer and then sends the packet to the slave, did not reduce the size of the send buffer as expected. (Bug #24643036)