What is the output of running npx envinfo --binaries --languages --system --utilities?
System:
OS: macOS 10.14.4
CPU: (8) x64 Intel(R) Core(TM) i7-4850HQ CPU @ 2.30GHz
Memory: 420.60 MB / 16.00 GB
Shell: 3.2.57 - /bin/bash
Binaries:
Node: 8.11.1 - /usr/local/bin/node
Yarn: 1.3.2 - /usr/local/bin/yarn
npm: 5.6.0 - /usr/local/bin/npm
Utilities:
Git: 2.18.0 - /usr/local/bin/git
Languages:
Bash: 3.2.57 - /bin/bash
Java: 1.8.0_60 - /usr/bin/javac
Perl: 5.18.2 - /usr/bin/perl
PHP: 7.2.7 - /usr/local/bin/php
Python: 2.7.15 - /usr/local/bin/python
Ruby: 2.3.7 - /usr/bin/ruby
What are the steps to reproduce?

1/ create color rectangle with { r: 127, g: 255, b: 127, alpha: 1 } to use as the source
2/ create semi transparent rect with { r: 0, g: 0, b: 0, alpha: 0.8 } to use as the destination
apply the source to the destination by composite with blend mode "in"

What is the expected behaviour?

the outcome should have the color of the source and the alpha of the destination { r: 127, g: 255, b: 127, alpha: 0.8}, instead, the result is { r: 158, g: 255, b: 158, alpha: 0.8}.

Are you able to provide a standalone code sample, without other dependencies, that demonstrates this problem?

sharp({
create: {
width: 300,
height: 200,
channels: 4,
background: { r: 127, g: 255, b: 127, alpha: 1 }
}
})
.png()
.toBuffer()
.then(colorPanel=>{
sharp({
create: {
width: 300,
height: 200,
channels: 4,
background : { r: 0, g: 0, b: 0, alpha: 0.8 }
},
})
.composite([
{
input: colorPanel,
blend: 'in'
}
])
.toFile('img/test.png')
})

Are you able to provide a sample image that helps explain the problem?

All the images can be created with code