The new comment threads, recently introduced in windows excel, are not compatible with the latest 2019/office 365 excel program.
Excel needs to be compatible between platforms. Unfortunately we are now experiencing data loss because of the approach Microsoft used to implement the threaded comments capability.
The customer switched over to Insiders fast and saw the problem resolved.
Paul Gustafson commented
What’s really crazy is that the iOS rev of Excel shows the threads better than OSX!!! What is Microsoft thinking????