Engine sensors can certainly have an impact on transmission operation, especially the MAF sensor, and going through a carwash could have picked up some a lot of extra moisture/water in the intake system depending on what you have on your car and can cause the maf sensor to read wrong. The pcm uses MAF signal to determine engine load based on airflow across the sensor and to some extent uses this information for line pressure, but at part throttle conditions it wont have near the effect as WOT conditions. While at very low throttle positions or shifting into gear the pcm has base pressures that it commands to do the job and it an engine sensor or wiring issue likely wouldnt cause slippage, though it could cause max line pressure which would give very harsh engagements into gear and during shifts.
