diff --git a/HISTORY.rst b/HISTORY.rst
index c8949179..0d914a36 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -15,7 +15,6 @@ PlatformIO Core 5
 
   - Upgraded build engine to the SCons 4.1 (`release notes <https://scons.org/scons-410-is-available.html>`_)
   - Fixed an issue with Python 3.8+ on Windows when a network drive is used (`issue #3417 <https://github.com/platformio/platformio-core/issues/3417>`_)
-  - Fixed an issue when "strict" compatibility mode was not used for a library with custom "platforms" field in `library.json <https://docs.platformio.org/page/librarymanager/config.html>`__ manifest (`issue #3806 <https://github.com/platformio/platformio-core/issues/3806>`_)
 
 * **Package Management System**
 
diff --git a/platformio/builder/tools/piolib.py b/platformio/builder/tools/piolib.py
index 792d6fab..f6b9824b 100644
--- a/platformio/builder/tools/piolib.py
+++ b/platformio/builder/tools/piolib.py
@@ -773,19 +773,11 @@ class PlatformIOLibBuilder(LibBuilderBase):
 
     @property
     def lib_compat_mode(self):
-        missing = object()
-        mode = self.env.GetProjectConfig().getraw(
-            "env:" + self.env["PIOENV"], "lib_compat_mode", missing
-        )
-        if mode == missing:
-            mode = self._manifest.get("build", {}).get(
-                "libCompatMode",
-            )
-        if not mode and self._manifest.get("platforms"):
-            mode = "strict"
         # pylint: disable=no-member
         return self.validate_compat_mode(
-            mode or LibBuilderBase.lib_compat_mode.fget(self)
+            self._manifest.get("build", {}).get(
+                "libCompatMode", LibBuilderBase.lib_compat_mode.fget(self)
+            )
         )
 
     def is_platforms_compatible(self, platforms):