Steps to reproduce
Run GeoPackage through 1.2 executable test suite to confirm it is valid
Load GeoPackage into QGIS to confirm that it works when loaded directly from a file
Add new store, GeoPackage (mosaic)
Name it, identify a file on file system
When prompted, publish its "imagery" layer
Layer Preview, pick layer, choose JPG
Alternatively, load layer into QGIS as a WMS or WMTS layer
Layer preview shows something resembling the layer
Layer can be loaded into QGIS properly
Layer preview shows up blank/white
When loaded into QGIS, it shows up blank/white
As far as I can tell, the issue is because this tiles layer has global extents but is sparsely populated (with the bounding box identified in gpkg_contents). This test file has six tiles in it (to make it minimally reproducible). Note that cat tiles were used to avoid copyright issues.
GeoPackages with non-global extents (e.g., https://portal.opengeospatial.org/files/?artifact_id=74983) do work as expected.
I'm on MacOS Sierra (12.2.6) but this problem was first identified on another platform so I doubt that is the issue.