Ok the plot has most certainly thickened!
After fighting with this again I put a post up on stackoverflow and a helpful person pointed out that for some reason adding an link to an inline list item made the ends of the rows lose their borders. Even they had no idea why this jiggery pokery happened but it did. Though it didn't work in Firefox only chrome.
I was able to recreate this in Webflow pretty quickly. Unfortunately it didn't work for me as my in-line list items now flowed great but they would break on spaces bwteen words in list items themselves.
The solution was to set the link to inline-block and set the list item to inline. While I understand the rules for inline content are very different to other display types I do not fully get why the border is not being rendered for the end elements.
This is the final solution, it works exactly as I needed it to though I would be more satisfied if I understood why it does this.