Gould's guide to music notation Behind bars says the following (p. 325):
An octave sign applies to all parts on a stave. A note written with an octave transposition should not share a stave with a note to be played simultaneously at pitch (loco) if there is a feasible alternative layout. Transfer either the ottava or the loco pitches to another stave. If necessary, add a third stave for this.
Where there is not room for a third stave, the occasional octave-transposed note may be placed on the same stave as notes played at pitch, provided that the extent of octave transposition is made absolutely clear. For clarification, extend a dotted line vertically to encompass occasional octave-transposed pitches [...]
Thus, if the notes you showed were the only ones played at that time, you have (at least) two options:
- apply the ottava bracket to all notes under it, or
- move the bottom part into the bottom stave.
If these are not possible for some reason (e.g. because there is separate activity in the bottom stave for the left hand), you should either:
- use a vertical extension line¹ down from the ottava line, or
- use a third stave.
¹I tried to make an example of this, but it turns out to be pretty hard to convince Sibelius to do this. An example is given in Gould's book (p. 325).