Hi there! You could use a formula field for this, and the SUBSTITUTE() function, which could remove those added spaces in its output:
SUBSTITUTE({insertYourFieldNameHere}," ","")
Since it looks like the linked records’ primary field values you’re working with don’t have spaces " " in them already, this should help with what you’re aiming to do here – but note it’ll replace any space in your field values.
Just make sure to replace {insertYourFieldNameHere}
in the example above with the field name in your base
Here’s what it could look like:

Hi there! You could use a formula field for this, and the SUBSTITUTE() function, which could remove those added spaces in its output:
SUBSTITUTE({insertYourFieldNameHere}," ","")
Since it looks like the linked records’ primary field values you’re working with don’t have spaces " " in them already, this should help with what you’re aiming to do here – but note it’ll replace any space in your field values.
Just make sure to replace {insertYourFieldNameHere}
in the example above with the field name in your base
Here’s what it could look like:

Hey!
That is perfect, thank you very much!
Hey!
That is perfect, thank you very much!
One thing to be aware of is that if any of your single select items contain spaces (which is valid), those will also be removed. A more precise approach with fewer possible drawbacks is to change the comma-space combo into an empty string:
SUBSTITUTE({Field Name}, ", ", "")
In your screen capture, it looks like you have a linked record field, not a multiple select field. It also looks like you are copying from the linked record field into a single line text field.
You can use a rollup field with the ARRAYJOIN
aggregation formula, and you can specify whatever separator you like, including a single comma without a space.
I also just wrote a script that will copy the value from a linked record field to a text field or a multiple select field.