Decls added with `RBS::Environment#<<` disappear after `RBS::Environment#resolve_type_names` in RBS v3 or later

Decl added using RBS::Environment#<< will disappear from within env after RBS::Environment#resolve_type_names.
But, Decl added using RBS::Environment#add_signature will remain.
Is this intended behavior?

p RBS::VERSION
#=> "3.0.4"

env = RBS::Environment.new
_, _, decls = RBS::Parser.parse_signature("class Foo\nend")
env << decls.first

p env
#=> #<RBS::Environment @declarations=(1 items) @class_decls=(1 items) @class_alias_decls=(0 items) @interface_decls=(0 items) @type_alias_decls=(0 items) @constant_decls=(0 items) @global_decls=(0 items)>

p env.resolve_type_names
#=> #<RBS::Environment @declarations=(0 items) @class_decls=(0 items) @class_alias_decls=(0 items) @interface_decls=(0 items) @type_alias_decls=(0 items) @constant_decls=(0 items) @global_decls=(0 items)>

buffer, directives, decls = RBS::Parser.parse_signature("class Bar\nend")
env.add_signature(buffer: buffer, directives: directives, decls: decls)

p env
#=> #<RBS::Environment @declarations=(2 items) @class_decls=(1 items) @class_alias_decls=(0 items) @interface_decls=(0 items) @type_alias_decls=(0 items) @constant_decls=(0 items) @global_decls=(0 items)>

p env.resolve_type_names
#=> #<RBS::Environment @declarations=(1 items) @class_decls=(1 items) @class_alias_decls=(0 items) @interface_decls=(0 items) @type_alias_decls=(0 items) @constant_decls=(0 items) @global_decls=(0 items)>

Reproduce the issue using RBS::Environment#<<, RBS::Environment#resolve_type_names, and RBS::Environment#add_signature with the signatures shown in the report. Inspect the environment state before and after resolution, then determine the intended behavior for declarations added through both APIs and add coverage for the chosen behavior.