Changing style in parts should activate the "Apply to parts" button.
Frequency
Many
⇒
Once
Severity
S4 - Minor
⇒
S5 - Suggestion
Workaround
No
⇒
Yes
This is only available if you are in a part. When you create parts, they adopt any styles in the main score. Once you create parts, adjusting the main score's style never affects the parts. So the "Apply to parts" button is only available in parts.
I can see where you may want all parts to be changed by changing the main score, so the "Apply to parts" button should be available if there are parts.
I have changed your bug to a suggestion and improved the title for you.
Comments
This is only available if you are in a part. When you create parts, they adopt any styles in the main score. Once you create parts, adjusting the main score's style never affects the parts. So the "Apply to parts" button is only available in parts.
I can see where you may want all parts to be changed by changing the main score, so the "Apply to parts" button should be available if there are parts.
I have changed your bug to a suggestion and improved the title for you.