The 8th seems to ring a bell. Will see if I can dig through stuff to find out for you.
Edit. Yes, the 8th. I have two in my Amazon order list and both say the 8th. Almost as good value as my business sims!
Also my reading of the scancom listing is the O2 sim start and refresh date is due to when the user activates it. Might be wrong. The thread OP was about Three sims.
I bought a 500gb/m esim from Scancom on the 4th of November with an expiry date of the 8th of October 25. I received an Welcome email from Three telling me my number within an hour of activating it and another at 9:46 am on the 7th telling me I had started a new "billing cycle" so I would say the reset is on the 7th. Never had a notification of a reset with any other sim from Scancom