Invalid Data Source for some selections?

Hello everyone,

I aim to create a single GeoJSON layer containing all countries' administrative boundaries (for now only level-2 and only land areas) with custom data fields such as ISO country codes and more, which I plan to extract from tags. Later, I will create a vector tileset for web mapping purposes. Since this is quite a large and resource-intensive request, I decided to handle it by downloading multiple countries in batches (around 20 at a time), and later merging the data from QGIS. 

Initially, the process worked well. The first downloads opened correctly in QGIS, so I continued using the same approach for additional batches. However, I am now encountering that some files simply won't open. According to QGIS, the data source is invalid. I looked at these GeoJSON files' code and they seemed right. I validated them using external tools (https://www.itb.ec.europa.eu/json/geojson/upload) and they appear to be valid, so I’m unsure what might be causing the issue (I used the same request settings for all of the requests...).

So far, I’ve tried several troubleshooting steps, including changing settings during the download, downloading the data in other formats, or reducing the selection size. Despite these efforts, the issue persists. I found that certain countries, such as the United States or the United Kingdom, won't open, even if downloaded alone. 

Simplifying the geometry to 0.001 looks like a a workaround to bypass this issue, but I need high-resolution data (0.0001 ~ 10m).

Has anyone else experienced similar problems or have suggestions on how to resolve this?

Thanks in advance for your help!