Weird paging behaviour when chat has between 40 and 60 posts

[FelberMartin]

Describe the bug

When a chat has between 40 and 60 posts, the paging of posts does not retrieve the posts in the correct order. This messes with the indexing of posts introduced in #375, and in turn wrongly displays the unread posts indicator.

To Reproduce
Steps to reproduce the behavior:

1. Go to a chat with > 40 but < 60 posts (TS2 > "notificationsbusters" between test_user_1 and test_user_2, or create a new group chat)
2. In MetisChatList, add a simple Text that displays the index of the ChatListItem.IndexedPost
3. Go to that chat
4. See that the indexes do not start at 0

Expected behavior
The indexes should be in order.

Screenshots

Additional context

After a refresh, or re-entering the same chat, the indexes behave correctly again.