diff --git a/src/tests/execution/process_pty_test.py b/src/tests/execution/process_pty_test.py
index 80260ae5..f724a4d0 100644
--- a/src/tests/execution/process_pty_test.py
+++ b/src/tests/execution/process_pty_test.py
@@ -1,5 +1,4 @@
 import os
-import threading
 import unittest
 from pathlib import Path
 
@@ -9,6 +8,22 @@
 from utils import file_utils
 
 
+def _run_and_read_all(process_wrapper):
+    """Start the process and return its full output, deterministically.
+
+    The output is read on a separate thread (started by start()) that pushes
+    chunks into a ReplayObservable and closes it once reading is done. Reading
+    the output via read_until_closed() *while* that thread is still pushing races
+    the (unsynchronised) ReplayObservable.subscribe vs push and can drop the last
+    chunk — the source of the flaky test_PYTHONUNBUFFERED failure. Waiting for the
+    stream to close first means there is no concurrent pusher, and close() keeps
+    the replay buffer, so the late subscribe replays every chunk.
+    """
+    process_wrapper.start()
+    process_wrapper.output_stream.wait_close()
+    return ''.join(read_until_closed(process_wrapper.output_stream))
+
+
 class TestEnvironmentVariables(unittest.TestCase):
 
     def test_default_variables(self):
@@ -38,11 +53,7 @@ def execute_and_get_passed_env(custom_variables):
         env_variables = test_utils.env_variables
         process_wrapper = PtyProcessWrapper(
             str(Path(__file__).parent.parent / 'scripts' / 'printenv.sh'), '.', env_variables.build_env_vars(custom_variables))
-        process_wrapper.start()
-        thread = threading.Thread(target=process_wrapper.wait_finish, daemon=True)
-        thread.start()
-        thread.join(timeout=0.1)
-        output = ''.join(read_until_closed(process_wrapper.output_stream))
+        output = _run_and_read_all(process_wrapper)
         lines = output.split('\n')
         env_dict = {line.split('=', 2)[0]: line.split('=', 2)[1] for line in lines if '=' in line}
         return env_dict
@@ -54,13 +65,7 @@ def test_many_unicode_characters(self):
         test_utils.create_file('test.txt', text=long_unicode_text)
 
         process_wrapper = PtyProcessWrapper(['cat', 'test.txt'], test_utils.temp_folder, {})
-        process_wrapper.start()
-
-        thread = threading.Thread(target=process_wrapper.wait_finish, daemon=True)
-        thread.start()
-        thread.join(timeout=0.1)
-
-        output = ''.join(read_until_closed(process_wrapper.output_stream))
+        output = _run_and_read_all(process_wrapper)
         self.assertEqual(long_unicode_text, output)
 
         self.assertEqual(0, process_wrapper.get_return_code())
@@ -73,13 +78,7 @@ def test_mixed_encoding(self):
                               byte_content=True)
 
         process_wrapper = PtyProcessWrapper(['cat', 'test.txt'], test_utils.temp_folder, {})
-        process_wrapper.start()
-
-        thread = threading.Thread(target=process_wrapper.wait_finish, daemon=True)
-        thread.start()
-        thread.join(timeout=0.1)
-
-        output = ''.join(read_until_closed(process_wrapper.output_stream))
+        output = _run_and_read_all(process_wrapper)
         self.assertEqual('gültig\n läuft verändert für Ändern \nPrüfung gültig läuft ࠀ 𒀀!', output)
 
     def setUp(self):