Here is a thread that documents workaround with moving the messages between folders:
There is also a related feature request:
The reason there is no such function is that the result of downloading old messages is not very nice: Delta Chat generally places downloaded messages below the last read messages so you cannot miss them and everywhere we assume that messages are processed in the chronological order as they are received, e.g. when we receive “Member added”, “Member removed”, group avatar changes and so on.
If we want to do it properly, we should find the lowest message UID that we know and download batches of messages with this UID-100 to this UID, process them ignoring any group member changes and key changes and place them where they are expected to be according to the Date header. This is a different way of processing the messages if we want to make it a nice user function rather than a workaround for an emergency situation.
As we don’t know which message belongs to which chat, we also cannot download only messages belonging to the chat when you scroll to the top of it and want to see more history of this particular chat. Best thing we can do is “download N more old messages” and sort them into the chats.
If you have another device which has history still saved such as your desktop, best thing to do is to transfer a backup from your desktop to the phone. Or you can restore your old backup, it will then download new messages in the correct order.