It could be that simple-date is being accidentally (or intentionally) loaded. Could you check whether the :simple-date package is present? That will register handlers for serializing date and time values, which might cause the behaviour you describe. These two patches might be to blame:
https://github.com/marijnh/Postmodern/commit/f35a1124476d39d651baddbe1ba6791... https://github.com/marijnh/Postmodern/commit/ce1fd7a23b2e523dcc7b3f96fd60b48...
Best, Marijn