The same question, I don't understand, in Vietnam, Nokia 8 T-1004 (global dual sim) isn't received Update 8.1, We only receive Security Patch March 2018 (00WW_4_390_SP03) but my friend in Vietnam bought Nokia 8 in 10 April 2018, he received Update Android 8.1 with Build (00WW_4_84A_SP01). What is happening ? Why ? We bought Nokia 8 Global (free sim) and are in Vietnam, we receive other update
My TA-1052 Nokia 8 received March & April (though having error while installing) security patch timely but not the Oreo 8.1. Because of not able to install the Apr'18 security patch successfully, I google around and found a trick to downgrade the OS to Oreo 8.0 with Dec'17 security patch. After successfully downgrade, I receive the Jan'18, Feb'18, Mar'18 and finally Apr'18 security patches in each restart and have updated successfully.
Today, I thought of using the similar trick to update to Oreo 8.1 and it got updated successfully! You may want to refer to the steps I outline in xda-developers site for reference.
Do be reminded that the method works on my device, it may or may not work on your, do it at your own risk.
Nowadays, Nokia is releasing 8.1 via OTA.