The automatic grid has been effectively disabled with Affinity Photo 2 choosing a hardcoded 1px grid, ignoring my 10px subdivisions.
Replicated my settings under "Advanced," though I fail to see how making such a hardcoded change to the "Automatic" mode benefits anyone.
With the advanced setting, one can specify a 1px grid line with 1 subdivision and get results at 100% zoom rather than the arbitrary 800% zoom.
The 800% zoom limit makes the program appear to have a bug, and since there is no good reason for it, it is a bug.
For example, if at 100% zoom the grid lines appear too crowded, then the user has the choice of specifying courser grid lines or greater zoom level.
On the other hand, if at 100% zoom no grid lines appear, the user supposes that grid lines don't work, and justifiably so.