@Uncle Robin's issue should be fixed in today's nightly. (And, assuming your screensaver mode supports portrait orientations (which... they all should ;p), the reboot/poweroff screen will now also follow the lefty Portrait orientation instead of hard-resetting to canonical Portrait).
I... still don't understand @BiliBug's issue, because the screensaver should already be following the lefty Portrait orientation, so nothing should be rotating anywhere... Unless you're using a funky screensaver mode or were reading in Landscape?
(But, to answer the final question: yes, we can't actually ask the system "what's the *current* orientation?"[1], we only get "I just rotated <which way>" events as they happen, so we can't really resync to the actual orientation on resume, we have to go back to what we think it used to be *before* suspend).
[1] Well, it's technically possible, but it's a whole other kettle of fish, as it involves directly bitbanging the gyro over I²C

.