Hey, this is a known issue. The built-in browser tab grabs Option+Arrow after you interact with the URL bar, especially after picking something from autocomplete.
Duplicate bug report: Browser tab reacts on keys pressed in agent window
Workaround: close the browser tab and open it again. Option+Arrow will work as word-jump in the input again. The issue only comes back after you pick a URL from autocomplete again.
The team is aware and the bug is logged.