This might be a bug that was introduced in the 9/29 AGOL update: We weren't having problems earlier this week and we started seeing issues yesterday. I am curious if anyone else encounters this problem.
Our workflow is uploading various Excel files to become a hosted feature layer or tables. We have a dashboard where users upload daily Excel files that come from an outside source. I'm not sure if data formats beyond Excel have the same problem.
These Excel tables that have less-than-ideal attribute column header names that start with "bad" characters: some start with leading space, some with _ and some with numbers 0-9. Generally not good database design. Prior to 9/29 these uploaded fine and worked with the web maps and apps. If there was a space AGOL automatically added an underscore to the hosted feature layer name. All other ones did not give us errors.
Starting today, AGOL is automatically adding an "F" character before these characters. This causes the web maps and apps to break because they can no longer find the original column names.
We decided to change our column names prior to publication to remove the spaces and numbers and have adjusted our apps. But I can see the leading number example being a particular problem for some users. Also, it is possible there are other bad characters that will cause this problem.
Attached are 2 picture examples of the column names with the added "F" in our REST services. Note that the "F" is not added to the alias, only the column name.