Hi Daniel,
Greetings! Thank you for posting to Microsoft Community.
Have you tried change the text format to another one? It should be correct the display of it.
Most of this issue could be caused by encoding, but in cells it show correct value. So it should be text format settings on your workbook.

Best Regards,
Snow Lu
Hi all,
I had the same issue but ONLY with Arial font. All other fonts seemed to show the correct values in the cells. I closed the online version, opened the original in the Excel app, did select all and changed to another font (I chose Calibri 10 instead of Arial 8). After saving the original in the app, the online version opened fine. I know this is not a great workaround for everybody but it will work in a pinch.
Thanks!
Di