Agree it is an interesting phenomenon which occurs even with my squib load at about 1400fps with 20.0 to 22.0grs Red Dot or Vectan AS powder, a small amount of powder by any means in the big case.
I suspect that it is the large amount of leade my Mauser has that causes this phenomenon and contributes to the poor grouping of plain base cast bullets. Obviously gas checks protect the bullet base and help bite into the rifling despite the long leade.

I note some other posters on the subject of cast bullets in big bores also report poor grouping of plain based bullets while otherwise very good performance of same or other bullets with gas checks.

The magazine length on my Mauser prevents longer seating of cast bullets bringing the bullet closer to the rifling. It is not practical to single load cartridges with increased OAL, the Mauser is strictly controlled round feed.