I understand T4 has two ADCs and they are nominally 12 bit, but in practice around 10 effective bits. I also understand that they use a fixed Vref which is the internal 3.3V, and that same power rail supports the rest of the chip which can draw 100 mA, although I expect it varies depending on what internal peripherals are active and what external GPIO loads draw. I expect a varying load causes some noise on +3.3V.

I suppose an answer will await my own testing, but just in case anyone has already looked into this: if you measure an accurate external Vref with one T4 ADC, at the same time as your measurement of interest on another input pin using the other ADC, can you do a ratio and get a useful improvement in accuracy? The idea being you might calibrate out any load-driven noise on the internal 3.3V being used as reference for both ADCs.