Ok, I managed to reproduce the bug using this change:
diff --git a/Lib/test/libregrtest/win_utils.py b/Lib/test/libregrtest/win_utils.py
index f0c17b906f..78429faa89 100644
--- a/Lib/test/libregrtest/win_utils.py
+++ b/Lib/test/libregrtest/win_utils.py
@@ -14,7 +14,7 @@ BUFSIZE = 8192
LOAD_FACTOR_1 = 0.9200444146293232478931553241
# Seconds per measurement
-SAMPLING_INTERVAL = 5
+SAMPLING_INTERVAL = 0
# Windows registry subkey of HKEY_LOCAL_MACHINE where the counter names
# of typeperf are registered
COUNTER_REGISTRY_KEY = (r"SOFTWARE\Microsoft\Windows NT\CurrentVersion"
I wrote PR 16550 to handle partial lines.