Adding notes to bottom staff adds rests to top staff

Edit: I solved it - it was a simple mistake. Somehow the "insert" button was toggled rather than the default. Please ignore the below.

I am using the "voices" feature in my treble clef (voices 1 and 2) and in my bass clef (i know to start with voice 1 again, not voice 3) but the moment I try to input notes in the bass clef staff, those notes are added to the measure - the "placeholder" rests remain and new rests are added to the same measure in the top staff. i end up with measures that have twice the beats in them that they should have!

I have not found any info anywhere on this phenomenon but I am sure it is simply a beginner's mistake. I just can't work out how to fix it.

Additionally, and I don't know if this is related, but I am using a pick up measure. Bizarrely, the final measure of the piece is a full measure although it should automatically be shorter (accounting for the pickup).