I thought of this question while reading another question here.
In both examples of suicide-by-phaser that I know about, the phaser gets vaporized too! Think of the homeless man in The City on the Edge of Forever after he steals McCoy's phaser.
And there's the example in the in video clip below where Captain Terrell kills himself in Wrath of Khan. It's from 55 seconds to 60 seconds.
Why doesn't the phaser just drop to the floor after these suicides? The phaser is vaporized too! Which means the beam must turn back on itself or something.
The answer to this question probably involves treknobabble nonsense, but I would like to know if there is an in-universe explanation.
Edit: CanadianGirlScout found another example of suicide-by-phaser that destroys the phaser too. It's at the 3:26 mark in this video clip.