Don't think this is possible. You first have to scale down to one instance. See https://docs.mendix.com/howto/monitoring-troubleshooting/debug-microflows-remotely
The debugger supports only debugging of single-instance environments. Multi-instance environments need to be scaled down to one instance before the debugger can be used. See Scaling Your Environment in Mendix Cloud v4 for more information.
Hello Wouter, debugging multi-instance apps is not supported for Mendix Cloud. On Github you mention “because we see some strange activities when running in this mode....”. Can you describe that strange activities result in?
In general, make sure you have proper logging in your system as well to follow the process. Each request can be sent to a different instance because CloudFoundry will balance the requests over all instances running, so you’re never sure which instance gets the request. Any state you need, will need to be synced via the database.
To continue reasoning about possible causes, it would also be good to know if you run on Mendix Cloud and which login type you use (e.g. are you using Mendix AppCloudServices).