This just sounds like a description of Matrix's key sharing mechanism? Messages are supposed to be unencryptable if you don't have the keys, and bringing online another device (or having all your keys pre-shared so you don't have to) is what provides the keys. If you want to avoid this altogether, the UI prominently advertises the optional encrypted key backup service provided by the homeserver, and various manual options for sharing keys.

If FluffyChat is not having this issue, it is probably overly eager to share encryption keys instead of allowing the user fine-grained access to control keys, which is successfully hiding the complexity of the ratchet encryption but potentially exposing the user to attacks to force the sharing of keys.

Edit: I was looking around. While Matrix is well documented, Element's documentation is poor because they expect you to figure things out from popups in the UI -- fair enough, unfortunately most apps are like this, and Element's popups have gotten a lot clearer. But I did find these two pages from a university that seem to serve well as "Element's missing manual". Worth a read if you are trying Matrix for the first time, because it discusses some things that can look like bugs but are really user error.

https://docs.matrix.kit.edu/en/settings/

https://docs.matrix.kit.edu/en/faq/