JsonSchema::DEFAULT is documented to be "only supported on the root-level. Defaults for objects need to be defined at their root level". This decision was made, because nested defaults are trickier to support, and can be easily self-contradictory (nested and root-level default might differ and provide arbitrarily many different default values, and such a conflict would need to be resolved in a predictable way).
However, Growth's SuggestedEditsSchema violates this rule, and attempts to use defaults in referenced items. For this reason, the defaults do not work as intended, and are not applied at all. This should be fixed by making SuggestedEditsSchema use defaults at the root level, as documented by CommunityConfiguration.