ipv6servers database is repeatedly missing server descriptors on first day of the month

I learned today that the Relays by tor version graph has several gaps on the first day of a month:

After querying the database a bit I found that it's missing quite a lot of server descriptors on those days. And whenever too many server descriptors are missing for a day, the graphs do not show any data rather than potentially wrong data.

I'm now dumping the database to a file to download it and look closer on my local machine. I want to find out what these missing server descriptors have in common. Maybe we can fix this, even for past months.