Really frustrating that saved Workspaces disappears in Microsoft Remote Desktop 10.9.0 (macOs 13.5.1). Not like this in previous MRD versions.
I opened a support ticket on this, and we did two things. Updated to the current release 10.9.0 (2140) and fix email feed discovery (you use the email address to get the feed instead of the URL). That appears to have fixed it for us.
If this helps, what we did to fix email feed discovery was to add a DNS record on the email domain of:
Type: TXTHost:_msradc
Text: https://rdweb.wvd.microsoft.com/api/arm/feeddiscovery
I hope this helps someone else.