That has to be a pretty recent development, because I have lived here for 5 years and I have never seen a number written like that.
Sometimes you will see numbers written with the corresponding kanji after every four digits, like 1億2345万6789, which I like because it quickly tells you the magnitude, but it is not very common.
I also see that mixed character-numeral format you mentioned pretty commonly here in Taiwan, especially with 萬 (simplified to 万 in Japan post-WWII). For really large numbers, there's usually not so much precision, so the tail part is just dropped.
Sometimes you will see numbers written with the corresponding kanji after every four digits, like 1億2345万6789, which I like because it quickly tells you the magnitude, but it is not very common.