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The evidence is all in this table:

Test details:

1 minute intervals comparing the power from crankset Quarq power meter vs 1 minute at Zwift ERG-mode interval set wattage.

Setup details:

Trainer = Tacx Flux S
Date of Purchase = January 2021 (trainer is ~2 weeks old as of posting)
Primary training platform = Zwift
Cassette = brand new Shimano 105 10 speed 11-28 (cassette on my wheel is Ultegra 10s 11-28)
Chain has ~500 miles on it.
Chain ring = 53/39
It took me a while to properly index the gearing on the new cassette. But now it feels pretty well indexed when in both big and small chain ring (maybe slightly better in the small than the large). I would say that the trainer is louder when riding in the big chain ring. It feels like there might be some amount of chain rubbing. When I spin the chain backwards in the big ring it seems like the chain slips a bit more than if I do the same motion in the small ring.

Background:

New to Zwift and direct drive trainers. But in the first couple races I did I noticed that I had a super tough time putting out wattage in the big ring. I thought maybe I just wasn't used to the trainer. But once I did several of the workouts in ERG mode I realized that hitting wattage targets in the small ring was way easier than in the big ring. For high wattage intervals it felt like I was putting 30+ extra watts when in the big ring. So I decided to collect some actual data. And as the table above shows, there is some evidence that at high wattage 300+ (and perhaps exponentially increasing) the trainer under-reports wattage.
I will note that for the data I collected I would roughly estimate I averaged ~65 rpm for the big ring intervals and ~75 rpm for the small ring intervals.

Questions and Concerns:

First, what is happening and why?
Second, is this a trainer issue or a bike issue?
Third, my biggest concern is how this data translates to racing my bike outside. Honestly I don't care if Zwift under-reports my data but I do care if I was somehow sacrificing 20+ watts in the big ring outside and I just never knew. Could this be the case?
I know that power meters at the rear wheel will read lower than those under foot or at the crank. But given that the power readings match when in the small ring I suppose this is not a transfer of power issue? Or maybe it is? Could the big ring cause the chain to move at an angle that somehow rubs the cassette more and loses 20+ watts?
Does cadence have anything to do with this?
Could I have installed the cassette on the trainer incorrectly? Is the indexing or other derailleur adjustments incorrect?
Is an older (stretched but not overly so) chain on a new cassette somehow responsible?
Why do the loses seem exaggerated at 300+ watts but more negligible at <200?
My Quarq power meter is drive-side on my 'stronger' leg. I don't actually know if there is any imbalance. But could imbalance be relevant?

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