Update from Miro support - appears to be expected behaviour at the moment.
Thank you for reaching out to Miro and raising the users' concerns with the paste style in the Mind map nodes. Rest assured, we'll be glad to assist!
Mind maps have lots of limitations at the moment. As I replicate the issue, it appears that there's no functionality to paste styles onto mind map nodes. The current functionality only allows copying and pasting the style of nodes to other objects; but, transferring styles from other objects to mind map nodes is not possible.
In contrast to sticky notes or objects like shapes, where there is an option to copy and paste styles. The application of colors from other objects to sticky notes/shapes is possible, provided the color is compatible with sticky notes.

This might be one of the limitations of the initial widgets the mind map is built out of. Currently, we can only suggest having the users upvote this request, so the Product team can look into this and prioritize these feature additions.
I understand that this doesn’t solve the users' request at this time, but hopefully, we'll get a chance to implement this feature soon!
I hope this gives clarity! Please feel free to reach out if you have additional questions.
Here is a Wish List post for upvoting: https://community.miro.com/ideas/paste-style-for-mind-map-nodes-17101