This question of a bar was being discussed in a forum when we test drove XC40s last year, so I made a point of feeling for it. I could detect a bar in the standard fabric seats of one car we drove but not so as to cause discomfort, but not in the Pro seats. We have the leather Pro seats with extensions on our present XC40 and obviously there is a ridge and gap when the seat is extended (I have it in the second of three positions) but I have not found it uncomfortable. I also suffer from sciatica, Glos Guy! I have the seat position such that I get light support of the backs of my thighs, not such that my legs hang off the seat creating pressure.
I forgot to mention that I find the tilt function on the Sport seats on our BMW to be very useful. I found that rather than having the under thigh pad extended, if I tilt the seat backwards very slightly (so that the front of the seat base raises) I get more than adequate under thigh support even with the pad fully retracted.