preinsert with autocomplete only inserts the longest completion #18314

Steps to reproduce

1. set completeopt=menuone,preinsert autocomplete
2. type some text

I thought it would be interesting to have auto-completion candidate preinserted when typing, but trying preinsert with autocomplete results in only the longest text preinserted, even though completeopt does not contain longest.

hello
hella
he[ll] <- preinsert stops at ll

Expected behaviour

I expect preinsert to preinsert the first completion when autocomplete is running if completeopt does not contain longest.

Version of Vim

9.1.1754

Environment

Windows 10

Logs and stack traces