A new caption with the default settings will use both Arial and Cambria Math.
The main pre-filled text is Arial, there is one space using Cambria Math. If the user starts typing any new text, this will automatically be Cambria Math.
This in itself in an issue, besides that both fonts are unavailable in the list on the OnlyOffice desktop application for Linux, which is basically a second bug.
OS version: Linux Fedora 41
App version: 8.2.1.38 (flatpack)
I also tried this on MacOS, but there this issue does not occur.