I assume your default font does not support cyrillic characters, which is why they're missing in the export.
Have you tried using a different font (e.g. Arial Unicode), which supports all needed characters?
My default font does support cyrillic characters but not sure if cloud installed fonts are. Without success tried to use CSS styles as described here https://confluence.atlassian.com/conf76/advanced-pdf-export-customizations-1018768533.html
My style is
Any ideas and help appreciated. Thank you!