The .mp3 used here seems quite normal to meIt's not that 7/4 is weird, it's just that in this case, there is no instance where the song goes into 3/4 or 7/4.
And. Multiple songs have weird x/4 setups
I once mapped a 13/4 with 11/4
It was vocaloid
A 7/4 for once isn't that weird imo
The song is 4/4 all the time. You can use BRADIO's official OP as proof.
Perhaps you got your mp3 from here, where the beat is cut off from the beginning.
I do want this beatmap to be ranked and stuff, but the missing beat at the beginning is probably unexpected for those who expect 4/4 timing.