Hi @robmeades could you try the new Release Candidate we just released? We made some modifications in the discovery manager and probably it solves your issue.
The discovery manager has been introduced recently in the Arduino CLI and, as its name implies, manages all the pluggable discoveries that are running. The discoveries are a new kind of tool that platform developers can create to add support for different board and protocols. By default the Arduino CLI has two builtin discoveries, serial-discovery and mdns-discovery, the former supports boards connect via a serial port, the latter boards connect on the local network.
You can find more detailed information in the official specifications: