It is surprising. From the moment the host leaves the room, and the signalling server is notified of it, the room should be accessible again.
Either there is a bug that wasn't revealed in several years of usage of the signalling server, either you possibly aren't handling host's disconnecting as efficiently as you think you are ?
Consider providing your project to allow investigation on how you are doing things and what may be going on.
If you run in to a bug or issue in Construct 3, please post it to the GitHub issue tracker here: