What is the title of the thread -250 "on transition" not 250 now?
But I think that is why some of us are confused. It seems from your posts that you think that it's OK to be flying faster than 250kts while in 'Mach mode', then let the FMC decide when it is going to change over to 'IAS mode' and only then wind it back to 250kts. As I said, the phrase 'on conversion' implies an
acceleration to the requested IAS as altitude reduces.
If you like, It's the reverse of the climb schedule, e.g. 320/M0.85, where after conversion to mach, the IAS reduces from 320 as you climb.
Going back to descent, if ATC asked you to "maintain 320kts after conversion" there would be no real issue as your current IAS would in all likelihood be less than 320. The problem with the FMC arises when 250kts is slower than the current IAS at top of descent.
So, it's not that we think 'you don't know how the aeroplane works', but rather that you might not know what ATC want?
All offered with the highest respect and in the spirit of constructive debriefing!