Does your UAT SSRS server have the webdings font installed?
That is the only time I've seen issues with fonts. We use barcode fonts in some of our reports and we needed to ensure the font was installed on the client machine (to render it to them) and on the SSRS server (to build the report with that font).
After installing the font, we needed to reboot the machine unfortunately. I've even seen it where just looking in the fonts folder caused the font to be "installed" and a reboot resolved the issue.
Since this is a UAT folder, I would check C:\windows\fonts and double check the font exists. If it does, try rebooting.