This afternoon I used the new functionality in the latest AGOL release to remove a domain from a string field on a hosted feature service. I made the change then verified the domain was gone by making a test edit in a web map. Afterwards, a user was preparing for offline field work so she downloaded the data in Collector (ios). She was still seeing the domain in the offline data, so I double-checked in the web map and also in Collector (ios, online and android, online) but the domain was gone, as expected.
So she deleted the downloaded map in Collector, and when she viewed the map via Collector online the data entry was as expected (no domain). I went through some additional troubleshooting with her (log out, log in, try again, etc.) to no avail. So I tried disabling then re-enabling sync but no luck. Then I tired disabling then re-enabling editing which seemed to corrupt the feature service. We could start editing, but any changes made to an attribute would immediately revert.
Ultimately I had to re-publish the data and re-create the webmap. I still have the "corrupt" feature service if anyone from ESRI wants to take a look.