I understand special characters may not be supported, but the way in which they are handled in SharePoint 2019 is inconsistent with previous versions. First, attachments which are named with special characters can be added to items in classic mode, which is not true in SharePoint 2013 or 2016. An error is then reported when the item is saved, but the item is saved successfully without the attachment. Second, if such an attachment is added in modern mode, the item can be saved with the attachment, which then cannot be viewed. Though, as you mentioned, it can be viewed by switching back to classic mode. This seems to suggest that attachments with names containing special characters can in fact be saved properly in SharePoint, so I'm not even sure why this restriction exists.
This behavior seems like a bug, particularly the part about not being able to view such attachments in modern mode. Is it possible to report this as an issue to the appropriate team?
Thanks,
Holly