I've found a minor problem that is just a major opportunity in disguise. :)
I've noticed a behavior in MindManager 2016 in which the built-in spreadsheet is unavailable (greyed out in the "Advanced" ribbon) for some topics. Until now I wasn't sure what was causing this issue. What I found out is that if you add ANY custom property to a topic, you cannot also add a built-in spreadsheet to that topic. However, you can still attach an Excel spreadsheet.
I'm not sure if this behavior is "by design" (i.e. due to some technical constraint or incompatibility between custom properties and built-in spreadsheets) or just a bug. In either case, it's not an ideal behavior.
What would really be great is if custom properties "showed much love" for the built-in spreadsheets. Ok, I'm being a bit sarcastic here, but it would be extremely useful if custom properties could actually reference values stored in the cells of built-in spreadsheets. This would open all kinds of doors in terms of allowing more complex dynamically-calculated custom property values. That scenario would could nicely augment the already existing, but somewhat limited, formulas (e.g. sum, count, etc.) available for custom properties.
That would be much nicer than the current situation which seems to be that custom properties refuse to co-exist on the same topic that has a built-in spreadsheet. I've posted a separate idea recently suggesting the ability of being able to access custom properties from Excel. But I'm guessing that getting MindManager's custom properties to work with built-in spreadsheets might be easier than the Excel idea. Of course, my assumption is that custom properties and built-in spreadsheet functionality is included natively within the MindManager application code and therefore completely accessible to Mindjet's developers (as opposed to Excel functionality which must be accessed through the Excel object model/API).