In addion, I found a similar issue and solution[1] is to call a
gnus-group-get-new-news-this-group with prefix or <C-u M-g> for shor on
a nnimap group with missing messages.

Oleg Pykhalov <go.wigust@gmail.com> writes:

> When I invoke gnus-group-select-group or gnus-group-read-group or
> gnus-group-quick-select-group on my INBOX group, Gnus asks how many
> articles I want to read.
>
> - If I type Enter it will say “No unread news” and makes group as read.
>   0: INBOX
>
> - If I type a small number, like 20, it will open 20 last messages.
>
> - If I type a number of messages in echo area minus 1, I'll get
>   1: INBOX
>
>   If I type a number of messages in echo area minus 2, I'll get
>   2: INBOX
>
>
> I found similar issue¹, but the solution was upgrading to latest Gnus in
> 2010.
>
> Any thoughts?
>
> ¹ https://lists.gnu.org/archive/html/info-gnus-english/2010-12/msg00181.html

Also I broke the thread, here is another 'solution'[2] (worse than at
the head of the message). 

Footnotes: 
[1]  https://lists.gnu.org/archive/html/emacs-devel/2017-12/msg00624.html

[2]  https://lists.gnu.org/archive/html/info-gnus-english/2017-11/msg00005.html