Another problem with the Delete page and non-standard "enhanced" UI that breaks the rules.
To reproduce the bug:
This appears to be working correctly, it is still collecting additional characters to resolve mutli-letter conflicts.
Example, on testwiki (https://test.wikipedia.org/wiki/MediaWiki:Deletereason-dropdown)
You can start type V, A, and if you type M you get "vamdalism" but if you type N you get "vandalism"
If you hit up arrow, or down arrow, you are back at 'letter 1' and can type a new first letter in
@Xaosflux is right. When you start typing, characters you type are collected and used to search within the dropdown. The search is reset (and you can start typing from the beginning) after you move keyboard focus elsewhere, or after 1.5 seconds of no typing.
As far as I can tell, this is the correct and expected behavior.
Also, if this is throwing people off, perhaps the 1.5s delay should be adjusted. As far as I know it is just completely arbitrary. If there's some research/documentation somewhere about the appropriate time for this, or about how different platforms do this, we'd be happy to apply it.