Gaps in a tile layer can be caused by an issue from the data source. If the problem is on the data provider's end, there's nothing you can do to fix it in QGIS. To check if the problem is with the data or only in QGIS, try to see the data through another viewer or program. In this case, since the layer is hosted by arcgis.com, it can be previewed there. Here's the link for that:
I found that by pasting the WMTS service URL into my browser, then cutting off the end of it, so it was just
If the data has the same or similar gaps when viewed through a different data viewer, it means the issue is on the end of the data provider. Contact the data provider and notify them of the issue. In this case the issue resolved itself, presumably because the data provider noticed the issue and fixed it.