Cells display unreadable characters in spreadsheets when opened in Excel Online or previewed using OneDrive, however they display correctly when viewed from the Excel desktop app.
The font within the affected cells is not able to be loaded in the browser when using Excel Online or previewing the document with OneDrive. This causes the font to change to Unicode text.
The document still contains the data, however the browser cannot display it correctly.
As the issue is with how the browser interprets the font used in the spreadsheet, your IT team will need to review why the browser is instead displaying Unicode text.