Perhaps this will be obvious to some, but hopefully it will help shorten the search I just went through:
Deleting any field that does not allow null values, while editing a feature service in ArcGIS Pro, will cause a breakage that will make it impossible to take that feature service offline in Collector. The following is an explanation of how I came to discover this fact.
I recently received the following message when trying to download a web map via Collector:
"Map download failed. Unable to create replica. Please check your parameters. Internal replica error."
In addition to being unable to download the map in Collector, I also could not export the feature service as a file geodatabase. After failing to find a solution to this myself, I searched the internet to no avail. I finally had to create a new feature service using a local copy of the original geodatabase, then add all of the more recent data.
Though I was able to find a work around, I still wanted to understand what caused the issue in the first place. After creating some test data, I was able to determine the deletion of any field that does not allow null values, while editing a feature service in ArcGIS Pro, will cause a breakage that will make it impossible to take that feature service offline in Collector and may cause problems when trying to export the feature service to a geodatabase.
Hopefully this will help others avoid this hang up in the future.
