I know that this is an old post, but I wonder how long you used the Fiasp/Lyumjev mix, and if you are still using it what ratio you are on?
I think I used it for 3-4 months or so (30% Fiasp and 70% Lyumjev), it appeared to help with a more constant BG and absorption of insulin, then eventually I had a a few weeks of unexplained highs and crashing lows (as my Loop threw more insulin in to get my BG to drop).
In the end I went back to trying 100% Lyumjev, which worked well for a few cannula changes, then I got highs again.
Finally worked out it was critical with Lyumjev (and potentially Fiasp) to do very frequent site changes, ensuring I was using areas of the body that hadn't seen insulin in a long time - side effect seems to be I don't get any pain/discomfort from the insulins now
That seems to have fixed my Insulin resistance problems and I'm happily running a Fully Closed Loop using 100% Fiasp - it kicks in quickly and handles the unannounced carbs without a problem - issues are exercise (helps a lot if I set a temp target an hour before, or eat when walking the dog etc), and tuning the loop so I don't go too low (less insulin allowed for the Dynamic ISF part of the loop)
Last few days was 90% TIR - 2% low, 8% high with no time in very low - getting there slowly with the right insulin and understanding my body absorption