I recently encounted this issue with CSV files on Excel iOS app (app version 2.74). When trying to open a CSV file located on iCloud Drive, OneDrive or device itself, the Excel requires to enable 'Connected services' setting 'Analyze content'. If you don't allow that (or reject making any setting), the CSV file opens as blank in Excel app. If you enable 'Analyze content' setting, then CSV file opens into Excel normally (in read-only mode, which ok and documented feature).
Issue is reproducable with any kind of CSV file. I personally made the most simple CSV file with my Excel Windows version having just 1 row and two columns with data 'Data1' and 'Data2'. Moved this CSV file to OneDrive and accessed it from ipad and iphone with Excel app with given results: a 'Connected services' popup comes and if reject that or going to setting and do not enable 'Analyze content', the CSV file kind of opens but is totally blank (though not looking as normal blank sheet) and not editable.
Thus this is either a documentation fault, or error in the Excel iOS app behaviour.
On Windows version of Excel, CSV files can be used normally as ever.
All other normal Excel formats can be opened normally on Excel iOS app version without any 'Connected services' popup.
I have a privately owned Office 358 subscription and am a normal private consumer.
I did contact Microsoft office Chat support today (caseId 1055025104) and agent also considered this is a bug in Excel iOS version but recommeded to write to this forum.
All my devices are up-to-date.
So the workaround is to enable 'Analyze content' in the 'Connected services' settings, but that is not mentioned in the documentation and in general I don't want Microsoft to analyze my content especially with such a primitive file content type.
A typical scenario for consumers could be similar to my case where I encountered this issue: I can monitor my home electricity consumption on electricity operator company's website from where data can be downloaded to my device as CSV files (no option to choose format) for my further processing.