All washing up liquids will leave a residue behind, the stronger the solution, the stronger the residue.
This residue isn't visible, nor can you feel it, but particles in the air will stick to it.
The specialist detergents some of us use are designed for window cleaning, if diluted properly they leave far less residue behind, ergo the windows remain cleaner for longer.
Washing up liquid (Fairy or any other make) has a different remit to window cleaning detergent, it's a far more powerful detergent and has to cope with way more grease, baked on food, starch and all the other things that fester in your sink overnight (well if you've had a skinfull you don't want to do the washing up straight away...do you?) than you are going to find on windows.
Is it weaker now? Personally I don't know, I use the Unger liquid, and I no longer suffer from dermatitis.....though as most of my work is now WFP, that is the most likely reason I no longer have those problems
Regards,
Ian