Looks like manage somehow breaks from from the eval:
1 (user) => (let ((environment (package/environment (find-package '(user) #t))))
(eval '(begin (display 1) (manage 'new-environment 'combinators) (display 2)) environment))
1
;Loading "sdf/common/overrides.scm"... done
;Loading "sdf/common/utils.scm"... done
;Loading "sdf/common/indexes.scm"... done
;Loading "sdf/common/collections.scm"... done
;Loading "sdf/common/memoizers.scm"... done
;Loading "sdf/common/predicates.scm"... done
;Loading "sdf/common/predicate-metadata.scm"... done
;Loading "sdf/common/applicability.scm"... done
;Loading "sdf/common/generic-procedures.scm"... done
;Loading "sdf/common/pretty-printer.scm"... done
;Loading "sdf/common/operators.scm"... done
;Loading "sdf/common/operations.scm"... done
;Loading "sdf/common/package.scm"... done
;Loading "sdf/common/predicate-counter.scm"... done
;Loading "sdf/common/simple-tests.scm"... done
;Loading "sdf/common/trie.scm"... done
;Loading "sdf/combinators/function-combinators.scm"... done
;Value: (combinators)
1 (user) =>