Breakthrough US is always on attack side

If you mean the US faction is the one that is always on the attacking side on certain maps... then no that is not a bug at all and is a part of the lore for that map.

If you mean you are playing from somewhere in the USA and are always put on the attacking side when you play Breakthrough... maybe?

US now is on attack side even on those maps where it was defending earlier: Exposure, Orbital, Kaleidoscope, Spearhead, Discarded

Yes, the Russian camp can no longer be chosen to join, and now it seems that the only option is to forcibly choose the American camp