This bug was acknowledged by Google and now has been fixed. Refer to https://groups.google.com/forum/#!topic/pagespeed-insights-discuss/by9-TbqdlBM
From Feb 1 to Feb 4 2019, PSI had a bug that led to lower performance scores. This bug is now resolved.
The headless Chrome infrastructure used by PageSpeed Insights had a bug that reported uncompressed (post-gzip) sizes as if they were the compressed transfer sizes. This led to incorrect calculations of the performance metrics and ultimately a lower score. The mailing list thread titled [BUG] [compression] Avoid enormous network payloads / Defer unused CSS doesn't consider compression covered this issue in greater detail. Thanks for Raul and David for their help.
As of Monday Feb 4, 5pm PST, the bug is completely addressed via a new production rollout.