Unity 2019.2.0f1: Terrain won't raise or lower after PC reboot

Hello,

Using Unity 2019.2.0f1 on Windows 10.

Before starting any terrain work, I moved the terrain to 50 on Y and made sure my terrain's Set Height was set to 50.
(Terrain Height, in the Terrain Settings panel, was 600 by default. I didn't change it.)

I made a basic terrain with some hills and some low areas.

Raise/Lower terrain worked fine. I then painted textures onto the hills and the ground.

When I shut down and restarted my PC, open Unity and attempt to use Raise/Lower Terrain again, it will not work. I've checked everything I can think of: nothing's locked and nothing is 'in the way' to prevent the terrain from being raised or lowered again.

Does anyone know if this is a bug with Unity 2019.2.0f1? If so, is there a workaround?
Thanks in advance for any input.