Thank you for your post.
The behavior you describe follows the expected format "rules" for text in FileMaker Pro.
An important detail of this behavior is described in FileMaker Pro Help > Home >Designing and creating databases > Editing objects on layouts > Formatting text, as follows:
"Field data that is formatted in Browse mode takes precedence over data formatting in Layout mode. For example, if you format field text as Bold in Browse mode, then switch to Layout mode and format the same field as Plain Text, the text will continue to display as bold.
Please let me know if I am misunderstanding your example in some way. As I see it, you would like to be able to use Conditional Formatting to "reset" the formatting of a field, but if that format was set to "italic" or "bold" as default, it will not be changed when conditional formatting resolves as "true" for that field.
What Gianandrea Gattinoni describes happens even if no text is directly formatted at the data level.
If the layout settings on the field specify Bold, conditional formatting cannot change it to Plain though, as Gianandrea has pointed out, the reverse can be done so we can usually work around this by reversing the logic of the conditional format expression.
PhilModJunk is correct.
Moreover, let consider that if in Layout mode you format a text with Bold, In Browse mode you can format the same text as plain.
That is not possible with Conditional Formatting.