Typo in the title I meant "You CAN'T ..."
Steps to reproduce:
Try to set up a custom search engine, you're forced to enter something as the suggestions URL. Even if you later delete the suggestions URL, you can't make changes to the actual search URL. It looks like you can and the window closes itself, but no changes are made.
No suggestions URL being needed to set up or change a custom search engine
Orion and macOS:
0.99.107-beta (WebKit 613.1.3) on Mojave