Is Signal or Signal Desktop really as secure as Snowden claims?

I ask this for the following reason:

I have enabled "disappearing messages" in every chat, but when I open the Signal desktop application, this message appears upon startup: "Loading messages… 20 so far." Depending on how long I haven't used Signal desktop, the number then increases, sometimes to over 1,000.

What does this message mean? Are the messages, which have already been deleted from the devices, stored on some kind of server, or could it simply be that the synchronization works more like a blockchain?

ADDENDUM (11.01.23) :

I've also noticed in the desktop application that while the messages disappear from the chat history, they are !NOT! deleted: If you enter text or a word in the search field that you've already used in the chat, you can read the supposedly deleted message in its entirety!

I have already contacted support about this, but have not received a response yet.

(Version used is "signal-desktop 6.0.1-1"; Source: official Linux repository of Manjaro ARM)