Combo Chart displaying some XY scatter markers but not others

Good evening Forum,

I have a Stacked Bar / Line / XY Scatter combo chart that I am manipulating to use as a scheduling tool. It has been working flawlessly so far - that is, until I have tried to insert the last two XY scatter points and show their corresponding markers.

I've attached an example workbook with sensitive data removed to help illustrate my point. All the markers are showing as expected except for markers 'Test 55' and 'Test 56'. They should both be circles (albeit overlapped on top of each other), but for the life of me I cannot figure out why they won't show.

Is this a limitation of markers within a series? Some research indicates that an XY scatter chart can plot a much larger number than the 51 markers I'm currently showing. Maybe the formatting is too complex for Excel to handle?

Any help would be greatly appreciated!