Probably because you modified the text rather than the style.
Go to the one you tweaked and right-click on the style you used. Choose to update the style from the typing.
Importance of Styles in Word
Why use Microsoft Word’s built-in heading styles? by Shauna Kelly

No. I definitely modified the style. When I go to other occurrences of the style in the document that haven't updated their formatting and then select anew that same style, the formatting gets corrected.

You can right click on the style box in the ribbon and select all instances of the style, then left click the same box to ensure that the update takes effect. I think you might have mentioned this in a post earlier today, so my apologies if that was you--just mentioning it for future reference though if others visit, as I've had the same experience.