Setting value of Post Object field returning false

I'm trying to build some custom interaction which would auto-select some posts in a Post Object field based on a previously filled-in ACF field. For this I run an on.change event in a JS file which then calls the acf.getField function and tries to set the value like this:

field = acf.getField('field_5da85ee6a41e9');
field.val([255])

Problem is that this returns a false. While debugging this, I noticed that selecting and de-selecting the option, allows the code to run successfully. Here is a screenshot of doing what the JS code does except in the console where, between the two val functions I manually selected and deselected the option with ID 255.

Is there anything I can do to resolve this? I've tested the same functionality with a Select field with manually typed-in options and this works without a problem.

Thanks!
Freek

Start by reproducing the Post Object field behavior with acf.getField('field_5da85ee6a41e9') and field.val([255]), then compare it with the working Select field case. Trace the Post Object field's JavaScript value handling and determine why selecting and deselecting first changes the result. Done means setting the post ID programmatically returns the expected value without manual interaction.