This design-time message occurs if the True or False resulting actions of the macro rules of the current DiaryGridLine includes has a large number of unchecked style options, as in the example below.

Suspiciously high number of un-checked options on the LEFT and a more likely scenario on the RIGHT where most options are set to neutral (don't care).
The settings on the left are often due to a misunderstanding of the meaning of the 3 possible states of the checkboxes attached to the various attributes settings.

Meaning of the 3 checkbox settings in the Macros Reulting Actions options
The settings on the left do not simply mean "add bold"; it means "add bold and remove non-breaking and remove italic and remove outline ... and remove subscript".
Clearly this is a much more systemic change than simply changing it to bold, and is the reason for this message.
Solution
If you want to ignore, leave unchanged, attributes, then the checkbox setting to use is the neutral one.

Topic 181995, last updated on 01-Aug-2020