Yes, I'm having that issue recently as well, but it's occurring in Field Maps and also Collector and web apps with an edit widget. I have noticed that if you save the value on the type field and close the editing session, when you reopen the edit window/widget all the domains are at least populated properly.
For Field Maps and mobile OS versions of Collector, if the type field is changed, none of the associated other fields have the domain updated and the templates for the subtypes don't apply to overwrite the values. The only exception seems to be on Windows 10 Collector, if you select to "reset" the fields in the warning box then it does work correctly.
For the web apps, it mostly seems to work except when switching back to the original value on the record. As an example, if the type value is set to 3, if I change it to 1, 2, 4, or 5, it works to update the other fields, but it won't change back to the 3 value subtype domains unless you cancel edits or save as 3 and reopen the edit pop-up.
The issue is happening for me on some hosted feature service views, where I modified the type fields in the service definition for the views from REST, since each of the views have different fields visible and subtypes applied for different scenarios. However, I created a new geodatabase in ArcGIS Pro, set some test fields, domains, and subtypes in Pro, and published that as a hosted feature service to AGOL. That published test service exhibits the exact same behavior even with all the additional new definition fields and however Pro writes the definition, so I don't think it's an issue with the service definition but maybe something with AGOL and/or the apps.
I would also like to know if anyone else has run into this or has a solution, but maybe it's related to the recent AGOL update in December as well?
