There is currently a Zoom limit in the Geometry Parameter which prevents users from zooming in further than a specific level like (z21 in Google). A default limit for Google Maps is reasonable but when using own custom WMTS layers users should be able to set this limit. This “feature” of the Leaflet source code behind this function is currently quite “limiting”.
So either the default zoom limit should be adjusted further in Leaflet for custom WMTS layers or users should be given the capability to do so.