Hello,
When you only use a single manual grid (0.7 mm) on the bounding box, the entire domain - including the electrodes - is discretized uniformly at that resolution. While 0.7 mm is reasonably fine, it may still be too coarse to accurately capture the very steep electric field gradients near the electrodes, especially for TI setups where interference patterns are sensitive to local field accuracy.
When you enable Automatic Grid settings for the electrodes, the solver applies local mesh refinement around those electrode regions. This typically results in a smaller minimum step size near the electrodes than your global 0.7 mm grid. That improved resolution leads to more accurate current density and E-field calculations at the source, which then propagates into differences in the resulting interfering field.
So the change you see is not an error - it’s actually a sign that the solution is sensitive to mesh resolution near the electrodes, which is expected.
Regarding your other question: Using a bounding box vs. explicitly listing all head tissues generally does not change the result if the grid settings are identical, because the box already encloses all tissues. It’s mostly a workflow/simplicity choice.
I hope this information helps.