Probably you can forget it, I´ve come to a point to say, I do not care any more, because they want you to proof, that they are doing something wrong, you should show packetcaptures where this can be clearly seen. But that´s not to easy, because you cannot forget a client completely in the dashboard, the hostnames are kept in there, even if you use the "forget client" feature. So you cannot use one client to perform different tests. mDNS Hostnames can only be overwritten by new mDNS information. This makes testing very difficult, as you always need new clients.
My assumption:
I think that code in software beeing used on the devices, is generating mDNS packets with hostnames in it. In this case, if you see this UUID like information in the dashboard as a hostname, it is not the operating system itself, but an application generating it. The problem here is: Meraki does not differentiate between "only" applications sending out some packets, that should not be analysed for the hostname to be displayed in the dashboard.
In general Meraki says, it is not an issue, its an isolated case:
Message from 11.09.2020:
"Greetings,
Thanks for your reply,
I've investigated this further with our internal product specialist teams and confirmed there are no reports of this issue currently and more so we would need to acquire further data from the case to troubleshoot this further."
So all of you, having problems, in real you don´t have no problems! Since a few month since creating this ticket and reading all the posts from users with the same issue, I can say, that they don´t really care.