When a dropdown with search selection is disabled, the search input is available with tab key
https://jsfiddle.net/dutrieux/pzh25rg1/

2.8.6
Fixed by #1606
See your adjusted jsfiddle here https://jsfiddle.net/lubber/gcweburm/
The same happened to disabled calendar fields, which the above mentioned PR also fixes 🙂
I would have thought you would have used the readonly attribute rather than disabled.
Using readonly will not prevent to enter the input field while pressing tab. In case of an empty dropdown field, this would autoselect the first entry when pressing tab again (and, even worse, in case of calendar, would still open the calendar popup)

However, i fixed it by using tabindex instead of disabled now. Same effect, but the value will be submitted as the input field itself is not disabled anymore
See your example using the tabindex approach here now: https://jsfiddle.net/lubber/gcweburm/1/