Light Cache data is saved in the resumable files, but the Irradiance Map (or any other GI map) is not and needs to be recalculated when resuming. The rendering should still continue from where it was stopped after the GI data is recalculated.
The Resumable Rendering option should be enabled before starting a new render, not only when resuming.
To start a new render, the .vrimg/.vrprog file should be deleted.
The scene should not be changed between stopping and resuming, except for the Progressive sampling options (Min. subdivs, Max. subdivs, Render time, Noise threshold).
Resumable Rendering does not support tiled EXR outputs.
On GPU, resumable rendering is only supported with Bucket sampler.
