Maybe stupid question... You do have the Ztella MQA version? Goole shows that this DAC comes in MQA and Standard version. Latter does not support MQA.

Secondly, in UAPP "settings" > "MQA" menu there is a checkbox "MQA passthrough". Do you have it checked or unchecked? If information on the Zorloo website is correct Ztella only is a MQA Renderer so this checkbox should be unchecked (you want the phone to do the first unfold). But in reality this should not be an issue because UAPP should detect DAC capabilities and act accordingly but maybe it is not for some reason and needs a bit of guidance.

Lastly, does the UAPP UI shows MQA at the bottom and is there green or blue dot next to it? If there are no dots then phone is NOT doing the first unfold and sends the raw file to the DAC. DACs without decoder will not be able to decode it and will treat it basically as simple 44.1/48 kHz file and play it "as-is".

That's all I can think of. But I think it may be also just specific issue with phone/UAPP/DAC combination. For instance my Zen DAC V1 never shows up magenta on my phone and UAPP, only green and yellow. But it works on PC so I guess it is just nature of the beast.

I typically spend a week with one headphone, then change it to another for another week. I mostly work from home so I can cycle them but I think that after listening to one set of cans for some time you need a bit of time to adjust to the new sound so giving them a week seems fair. Also, I can appreciate sound differences more as well as other nuances like build or comfort. I do not have any order specified, I just pick one at the beginning of the week, usually one that I haven't used for a while and also something different enough from the one I was using prior.