Dear Roberto from Canada
Good day! Thank you for posting in Microsoft Community. We are happy to help you.
Based on your description, it seems that you can't adjust the orientation of the object of the arrow and only can rescale arrows in OneNote version 16.66, but you can edit the arrows orientation in OneNote version 16.65.
Given this situation, we did a test on our end, and we found we can reproduce the same result as yours. In OneNote version 16.65, we can edit the length and direction of the line or arrow successfully, but in version 16.66, we only can edit the length.
OneNote version 16.65:

OneNote version 16.66

Given this situation, we kindly suggest you use OneNote version 16.65 as a workaround temporally. On another hand, since OneNote version 16.66 is a new release recently, it may have some limit features and we'll reported this behavior to the related team, and we suggest you send feedback by Select Help >Suggest a feature or Give feedback. to make our products and services better for you and others. It is the best platform for us to listen to your opinions and make our products better serve you and others. The user voting here can let us better understand what users want, so that we can better improve our products.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Stacey | Microsoft Community Moderator