Hi all,
Really thanks for all your effort to share this experience with us, in terms of your OneDrive Save button and Folders grayed out in iPhone and iPad issue, we’ve further investigate and we would like to add a summary of this to help other users get more information on such same situation.
Problem description:
OneDrive Save button and Folders grayed out when using Files app on iPhone/iPad.
Problem environment:
Microsoft 365 Apps or Office 365 Business
Suggestions:
Based on our internal resources, this is a temporary adjustment in IOS OneDrive and Files App entering read only mode which helps prevent future changes from being lost without proper save. This situation happens with OneDrive iOS version 12.18+ on iOS and iPadOS devices, the Files app experience will reflect that writes cannot be made at this time and will prevent end-users from editing files or folders from the Files app. End users will also temporarily be unable to save items to OneDrive using the "Save to Files" command in the iOS Share sheet.
Besides, you can also see the notice in OneDrive from Apple Store, like below screenshot:

Please notice the change is temporary, but we don't have an ETA when it will be reverted. There are discussions between Apple and Microsoft to determine how to fix. Given this case, we kindly suggest you be patient and wait for the newest updates.
Thanks all for your cooperation and understanding!
Have a nice day!
Regards,
Gloria