Class C transitions to Class A after "timeout" in application?

I have created an application that originally contained only one device of type “class c”. I was able to send messages (downlink) without problems, but if I waited for about 20 minutes “Enqueue” started saving messages as if it were a “class a” device.
To solve this I added a device that regularly sends messages (uplink), about once every 15 minutes, then “class c” works for the first device no matter how much time passes.
There seems to be a timeout in the application that I can’t find anywhere in the documentation. This timeout terminates the “class c” function if there is no other activity in the application for a certain amount of time. The timeout seems to be linked to the application as a whole and not to any specific device. Is there somewhere where this is described and is there any other way to keep “class c” enabled without having to send dummy messages?

Are you on the latest release of ChirpStack? There was a similar bug that was fixed a release or two back.