Are you seeing the scrambled texts in Confluence Powerpoint thumbnail like the below screenshot?
First of all, make ensure you have installed all right fonts in the OS or Docker image if you use containers.
Secondly, clean all caches and the temp folders like dcl-*, then restart Confluence.
If the above did not solve the problem. Then it is most likely caused by the java version, is your java version lower than Adopt OpenJDK 11.0.7_10? (This is a known bug: https://jira.atlassian.com/browse/CONFSERVER-59429)
For example, Confluence 7.4.1 binary bundle comes with Adopt OpenJDK 11.0.5_10. So it has such issue. The fix is to either upgrade the java version or upgrade the Confluence binary bundle to the latest version.
Confluence 7.4.3 binary bundle comes with Adopt OpenJDK 11.0.7_10 according to the matrix https://confluence.atlassian.com/doc/bundled-tomcat-and-java-versions-1005786018.html.