diff --git a/HISTORY.rst b/HISTORY.rst
index c7c25591..220a2576 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -2,6 +2,7 @@ Release Notes
 =============
 
 .. |PIOCONF| replace:: `"platformio.ini" <https://docs.platformio.org/en/latest/projectconf.html>`__ configuration file
+.. |LDF| replace:: `LDF <https://docs.platformio.org/en/latest/librarymanager/ldf.html>`__
 
 .. _release_notes_6:
 
@@ -72,9 +73,10 @@ Please check `Migration guide from 5.x to 6.0 <https://docs.platformio.org/en/la
 * **Build System**
 
   - Show project dependency licenses when building in the verbose mode
-  - Fixed an issue when `LDF <https://docs.platformio.org/en/latest/librarymanager/ldf.html>`__ ignores the project `lib_deps <https://docs.platformio.org/en/latest/projectconf/section_env_library.html#lib-deps>`__ while resolving library dependencies (`issue #3598 <https://github.com/platformio/platformio-core/issues/3598>`_)
+  - Fixed an issue when |LDF| ignores the project `lib_deps <https://docs.platformio.org/en/latest/projectconf/section_env_library.html#lib-deps>`__ while resolving library dependencies (`issue #3598 <https://github.com/platformio/platformio-core/issues/3598>`_)
   - Fixed an issue with calling an extra script located outside a project (`issue #4220 <https://github.com/platformio/platformio-core/issues/4220>`_)
   - Fixed an issue when GCC preprocessor was applied to the ".s" assembly files on case-sensitive OS such as Window OS (`issue #3917 <https://github.com/platformio/platformio-core/issues/3917>`_)
+  - Fixed an issue when |LDF| ignores `build_src_flags <https://docs.platformio.org/en/latest/projectconf/section_env_build.html#build-src-flags>`__ in the "deep+" mode (`issue #4253 <https://github.com/platformio/platformio-core/issues/4253>`_)
 
 * **Integration**
 
diff --git a/examples b/examples
index 5240b19d..cb4befb5 160000
--- a/examples
+++ b/examples
@@ -1 +1 @@
-Subproject commit 5240b19d2dcc76b566091c8b7ce0eeb8d485de5b
+Subproject commit cb4befb56b07a785907ef058738e0a4f1382dffa
diff --git a/platformio/builder/tools/piolib.py b/platformio/builder/tools/piolib.py
index 29976d62..55f1b7d1 100644
--- a/platformio/builder/tools/piolib.py
+++ b/platformio/builder/tools/piolib.py
@@ -900,13 +900,19 @@ class ProjectAsLibBuilder(LibBuilderBase):
         # pylint: disable=no-member
         return self.env.get("SRC_FILTER") or LibBuilderBase.src_filter.fget(self)
 
+    @property
+    def build_flags(self):
+        # pylint: disable=no-member
+        return self.env.get("SRC_BUILD_FLAGS") or LibBuilderBase.build_flags.fget(self)
+
     @property
     def dependencies(self):
         return self.env.GetProjectOption("lib_deps", [])
 
     def process_extra_options(self):
-        # skip for project, options are already processed
-        pass
+        with fs.cd(self.path):
+            self.env.ProcessFlags(self.build_flags)
+            self.env.ProcessUnFlags(self.build_unflags)
 
     def install_dependencies(self):
         def _is_builtin(spec):