You don't need invisible notes for this kind of situation; just use the proper syntax for polyphony, so that the upper voice remains in the main voice context:
<< { \grace bf16
\voiceOne %make stems point up for this voice after grace note cf2 af'4~ | %tie will continue outside polyphonic section } \new Voice { %second voice context created without using \\, so that first voice isn't in a new context
\voiceTwo %make stems point down here cf,2. | } >> af'8 %this note is now tied, with stem still pointing up \oneVoice %return to default voice behaviour
af[ gf f bf af ] |