This field, and the corresponding getter, and the code that extracts debug information from the NamedList response in extractDebugInfo, all use a Map<String, String> type.
However, when debug.explain.structured=true is used the values returned in response are not String-s, instead they are SimpleOrderedMap-s. This causes the following exception to be thrown:
The simple fix is to change the type of this field to Map<String, Object> but this would change the public API of QueryResponse.getExplainMap() in incompatible way. Still, I would argue it's worth to change it - AFAIK this getter is not used anywhere in the Solr's codebase, and the change makes it more consistent with other getters.