Not sure what's happening.

The bar# box came in versions newer than 2018 so ignore what I said about that. Viewing your bar settings picture, every thing looks in order. It appears that at Chorus 3, bar 1 the 434 fiddle should replace the synth on the soloist channel for all 32 bars of the third chorus and the synth should return on bar 1 of the fourth chorus.

I made a four chorus 8 bar following your instructions, 4 choruses with a synth soloist and the "mute melody in middle" box checked and I exited out.

I opened bar setting window and selected Chorus 3 at bar 1 and then the RealTracks button, selected the 434 fiddle and exited back to bar settings and the exited the bar settings window.

I generated the song using F4 and at bar 1 on Chorus 3, the fiddle replaced the synth for the entire third chorus and the synth replaced the fiddle at bar 1 of chorus 4...

So, it's working for me. Try saving the song, clear the chord chart and import the saved song again and see if that helps. I don't see any steps you're missing or doing incorrectly.

BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.