OP, I had/have perhaps a similar problem on my new 2020 T8.
Car worked fine for nearly 3 weeks after delivery. Took it in for "we owe" accessories to be picked-up and they found a new BECM Service Campaign that needed to be applied to 2020 T8's (all models) -- A39981. They did that Total Software Upgrade. All was fine on the drive home. Plugged-in for overnight charging which completed via my 240V Chargepoint wall charger. Next morning when I turned my T8 on, yellow engine check light came on after normal IC diagnostic checks. Tried the normal, turn off/on, short drive, etc 4 or 5 times -- same problem each time. Called dealer service when they opened, and they had me bring it in. Started the car to take it in, and this time, no check engine light, but I'd swear something odd was going on with the hybrid battery not engaging like it should -- the normal hybrid smoothness was replaced with how an ICE feels. Service found several "unusual" codes, but "no fault tracing", so after a test drive, cleared the codes and told me if the problem reoccurred they would have to give me a loaner, keep my T8 longer and they'd need to engage Volvo for further diagnostics.
In the past two weeks, I've had no more check engine lights, but do still think something with that software-only BECM Service Campaign either caused a problem, or by coincidence (I doubt it), some BECM/ERAD issue has started occurring. I'm trying to trace down a more easily described pattern to what I think is happening with what I'd call "my hybrid battery functionality temporarily disappearing", but will likely be calling to take it in next week... Net is, in Hybrid mode (I've not complicated this to try PURE since the problems began), I can back-out and start out fine, but even traveling slower speeds (<30mph), the right IC display will all of a sudden change as if there is no hybrid battery available to use (when I have 3/4 or more charge avail) -- display looks like it does when parked and ICE clearly is on for no apparent reason. The IC display may then again for no apparent reason while driving change to show the hybrid battery can be used again -- but oddly sometimes starts out at only a few marks up on the dial, then jumps to the more normal position around 11 o'clock where ICE will kick in. Odd. I've had it happen on the flat, going uphill, downhill, slower speeds and even driving along fine at 50+MPH where it drops back into ICE mode as I take lightly being to lift my foot off the accelerator a bit as one would normally do anticipating a stop ahead. Very odd. Something changed, and not for the good.
Good luck on your resolution!
Me too! 
