Dangerous implimintation of temporary traffic control by mobile units

Travelling toward N/Abbot on A38/A380 from Exeter I was warned by 3 advanced display vans (between Wobbly Wheel & the Shell Garage) that Lane 1 of 4 was closed ahead. I was travelling at 70mph with no other vehicles travelling west within some 300-400 m, at 21:30 12/05/23 in Lane 2, immediately after the split. There were 3 further Highway vehicles in Lane 1, all with flashing ambers, the rear one of which displayed a big Keep Right arrow, as I was fully expecting. What I was not expecting was for all 3 vehicles to pull out and across Lane 2 into Lane 3, the A38 ! Whilst I had to brake, not particularly heavily and avoid the convoy veering back into Lane 1, this could have been different if other traffic had been closer. Also I cannot understand how the advanced warning could be appropriately changed or not be misleading during this manoeuvre. This is something that should be addressed prior to the next such requirement. I am an Advanced trained motorcyclist and was on a fully HiVis marked fully illuminated BMW RT.