In Design / Borders / Width:
When adding a border width, un-toggle linked values, add 1 px as the right value and the border shows on the backend and frontend on 4 sides 1px. In order to get only 1 side, need to add 0 to each other side.
I would assume that in this case, the expected behavior would be a 0 value to any other side than the active one, right?
I don’t think what you are experiencing is a bug.
In CSS, the initial border width value is medium.
So you will have to set the other border widths to 0px in order to cancel the initial value.
Moving this to general.
Sadly not a bug, and can be unexpected for anyone that doesn’t know CSS well. It took me a while to realize this was what was happening.
I think the UX should be tweaked, since the setting of the border-style property applies a style to all 4 sides of an object. This conflicts with the next setting which is border width, which Cwicly allows us to set individually. What would make sense is to disaggregate border-style in the same way, since you can have different border styles per side. It could still keep the linking feature that we have for things like padding and border width so we can set one and it copies to the rest.
For such cases, tweaking or swapping the Normaliser is the way to go in my opinion.
I think there is a general confusion for many users, because the border-width property is set to zero in most tools/themes, etc.
I think that goes a step above what most users would be comfortable with, and is predicated on them having a solid base of CSS knowledge. I’ve learned a lot in the last 2 years, myself, but a lot is still beyond me. I can only imagine some others would struggle with it.