Weird regions hoping on zoom level 5

[xkomczax]

On zoom level 5, detailed region borders appears in many countries (e.g. Czech Republic, France, ...).
On zoom level 6, they disappear and instead superior regions boundaries appear (which are less detailed).
An example is visible on this map if you zoom in - instead of more details, you see less.

[pathmapper]

These boundaries are not included in the tiles on zoom level 6,7,8 and therefore could not be displayed.

Example:
https://www.openstreetmap.org/relation/7394

OSM admin_level = 6
NE 4.0 scalerank = 5 (changed to 3 with NE 4.1)
NE 4.0 minzoom = 8

On zoom level 5, these borders are included in the tiles from natural earth:
https://github.com/openmaptiles/openmaptiles/blob/master/layers/boundary/boundary.sql#L62-L64

On zoom level 6,7,8 these are missing because only admin_level <= 4 are considered:
https://github.com/openmaptiles/openmaptiles/blob/master/layers/boundary/boundary.sql#L71-L90

Starting with zoom level = 9, these boundaries are included again in the tiles:
https://github.com/openmaptiles/openmaptiles/blob/master/layers/boundary/boundary.sql#L92-L97

[eva-j]

Solved in openmaptiles/openmaptiles#613, discussion about zoom levels 7 and 8 in openmaptiles/openmaptiles#617.