The full story can be read in the issue report: Bundling base class and derived class in same dictionary can lead to failure in recursive parsing. · Issue #13034 · root-project/root · GitHub
To work-around the issue you just have to generate a rootmap file (to be kept in the same directory as the library). In the following the name of the rootmap file is A.rootmap (to match with the library name).
diff Makefile.orig Makefile
25c25
< rootcint -f ${DICT} -c ${DICTHDRS}
---
> rootcint -rml=A -f ${DICT} -c ${DICTHDRS}