Well that is weird as fish / mad as a box of frogs - have you ever tried swapping them over & seeing what happens - as I am sure that IS mounts are different spacings to rotor on front & rear
Just fitted some LX disc brakes with an avid 185mm adaptor on the front & a shimano 160mm front adaptor on the rear, seems to do the trick, rides okay so far as I can tell (I have hardly ever ridden a full suss bike) & pulls stoppies like a goodun. Looks a beauty though & a 1999 version so far as I can tell, so thats 12 years old & fresh as a daisy. Bit of a shame not to be able to keep the original formula brakes though
Not sure of the year at all, but looked like an early type of shock mount & vee brakes & forks point to 1999 & frame number has a 99 in it - but very willing to be corrected as I know f'all about these things