Starting with version 1.8, doxia-module-markdown changed from Pegdown to Flexmark.
However it seems that Flexmark does not support syntax highlighting.
doxia-module-markdown < 1.8 applied Java syntax highlighting here. doxia-module-markdown 1.8 does not. As doxia-module-markdown is used mostly for technical documentation in maven, I think this is a major regression.
Possible workaround: Use doxia-module-markdown 1.7.