What do you mean by “Versions after Oracle Java 8 are not supported since LibreOffice 6”? Start with clarifying which version is “6”? In LibreOffice, at least two first digits denote a major version; e.g. v.5.4 is a major release; 6.0 is a major release; 6.1 is equally major release. Then up to (including) version 6.2, the minimal supported version of Java was 5; for 6.3, minimal Java was 6. Starting with LibreOffice 6.4, minimal supported is Java 8; and versions after (i.e., newer than) the minimal supported versions are also supported.
It’s all described in the FAQ.