On latest master(29ed2c6e9a0c3cc57c5bbaf3c53e1cff8586c51c), sstate cache doesn't work even if the sstate mirror is set up properly. The bitbake doesn't take the sstate file when the checksum match
Root cause is that sstate's fetch code doesn't reflect latest fetcher changes, so old fetch style cause exception and fail silently. Fix will be out soon.
Fixed in master( 49a18f1748d2417958b8e19cdd58c0c79f4fc728) Thanks for Kevin, Lianhao & Qing's debug efforts!