Terrain Follow implementation

When I check the Terrain Follow box and Write WPs, then Read WPs, the Terrain Follow checkbox is then no longer checked. If it try to check it again I get a window saying "You will need to change your altitudes".

I have TERRAIN_ENABLE and TERRAIN FOLLOW set to 1.
When looking at the STATUS tab, I see ter_pend at 504 and ter_load at 0. I have a HUD display warning that says BAD/NO TERRAIN DATA.
How do I get the terrain data to load?

Start by reproducing the Terrain Follow, Write WPs, and Read WPs workflow while observing the STATUS values ter_pend and ter_load and the BAD/NO TERRAIN DATA warning. Trace the Mission Planner terrain-data handling from that workflow; done means terrain data loads and the Terrain Follow checkbox remains checked without the altitude warning.

Written by the indexing model from the issue text.