This is patch that might be used in https://github.com/conan-io/conan/pull/6014. Remove if merged.

multiple-values.patch

diff --git a/conans/client/cache/cache.py b/conans/client/cache/cache.py
index 52a6803f6..eede92fc9 100644
--- a/conans/client/cache/cache.py
+++ b/conans/client/cache/cache.py
@@ -124,7 +124,7 @@ class ClientCache(object):
         return join(self.cache_folder, ARTIFACTS_PROPERTIES_FILE)
 
     def read_artifacts_properties(self):
-        ret = {}
+        ret = []
         if not os.path.exists(self.artifacts_properties_path):
             save(self.artifacts_properties_path, "")
             return ret
@@ -137,7 +137,7 @@ class ClientCache(object):
                         raise Exception()
                     name = tmp[0].strip()
                     value = tmp[1].strip()
-                    ret[str(name)] = str(value)
+                    ret.append((name, value))
             return ret
         except Exception:
             raise ConanException("Invalid %s file!" % self.artifacts_properties_path)
diff --git a/conans/client/rest/client_routes.py b/conans/client/rest/client_routes.py
index 8719231c3..4bcee3b2c 100644
--- a/conans/client/rest/client_routes.py
+++ b/conans/client/rest/client_routes.py
@@ -22,10 +22,10 @@ def _format_pref(url, pref, matrix_params=""):
 
 def _remove_put_prefix(artifacts_properties):
     len_prefix = len(ARTIFACTS_PROPERTIES_PUT_PREFIX)
-    for key, value in artifacts_properties.items():
+    for (key, value) in artifacts_properties:
         if key.startswith(ARTIFACTS_PROPERTIES_PUT_PREFIX):
             key = key[len_prefix:]
-        yield key, value
+        yield (key, value)
 
 
 class ClientCommonRouter(object):
@@ -36,9 +36,9 @@ class ClientCommonRouter(object):
         self.base_url = base_url
 
         if artifacts_properties:
-            matrix_params = dict(_remove_put_prefix(artifacts_properties))
+            matrix_params = list(_remove_put_prefix(artifacts_properties))
             self._matrix_params_str = ";" + ";".join(["{}={}".format(key, quote(value, safe=''))
-                                                      for key, value in matrix_params.items()])
+                                                      for key, value in matrix_params])
         else:
             self._matrix_params_str = ""
 
diff --git a/conans/client/rest/rest_client_v1.py b/conans/client/rest/rest_client_v1.py
index 8fffac231..9da373971 100644
--- a/conans/client/rest/rest_client_v1.py
+++ b/conans/client/rest/rest_client_v1.py
@@ -156,9 +156,10 @@ class RestV1Methods(RestCommonMethods):
                 output.rewrite_line("Uploading %s" % filename)
             auth, dedup = self._file_server_capabilities(resource_url)
             try:
+                art_properties = {key: value for (key, value) in self._artifacts_properties}
                 uploader.upload(resource_url, files[filename], auth=auth, dedup=dedup,
                                 retry=retry, retry_wait=retry_wait,
-                                headers=self._artifacts_properties)
+                                headers=art_properties)
             except Exception as exc:
                 output.error("\nError uploading file: %s, '%s'" % (filename, exc))
                 failed.append(filename)
diff --git a/conans/client/rest/rest_client_v2.py b/conans/client/rest/rest_client_v2.py
index da9e08bf3..46a38036c 100644
--- a/conans/client/rest/rest_client_v2.py
+++ b/conans/client/rest/rest_client_v2.py
@@ -180,10 +180,11 @@ class RestV2Methods(RestCommonMethods):
                 self._output.rewrite_line("Uploading %s" % filename)
             resource_url = urls[filename]
             try:
+                art_properties = {key: value for (key, value) in self._artifacts_properties}
                 uploader.upload(resource_url, files[filename], auth=self.auth,
                                 dedup=self._checksum_deploy, retry=retry,
                                 retry_wait=retry_wait,
-                                headers=self._artifacts_properties)
+                                headers=art_properties)
             except (AuthenticationException, ForbiddenException):
                 raise
             except Exception as exc: