So I dug a little deeper...
The problem arises when manually setting the X-axis "from" and "to" properties (right click, properties, x-axis).
For some reason, the issue is when setting the date to 00:00:00 on the 1st day of any Year-Month (YYYY/MM/01 00:00:00). The date is saved in the .plc as 24:00:00 on the last day of the previous month - which the Plot Composer fails to read correctly. The fix is currently to add 1 second to the DateTime (or not to use the Day 1 @ 00:00:00).