Unable to manipluate string, read-only?

So I have some data which is a string, I'm passing it into a section with the following method:

{%- liquid
    assign myData = 'data],[data],[data'

    capture header
        section 'header'
    endcapture

    assign header = header | replace: '%%DATA%%', myData
-%}

Then within sections/header.liquid I have the following

{% capture replaced_data %}
%%DATA%%
{% endcapture %}

{%- liquid
    assign data = replaced_data | split: '],['

    echo data[0]
-%}

Now based off that, you'd expect the first data to be printed out, but infact the following is printed out

data],[data],[data

For some reason I'm no longer able to do any manipulation with this data once its in the sections/header.liquid file. I've even tried doing a replace on the ],[ but it just doesn't take effect. Any ideas on this?

Reproduce the behavior from the issue using the outer Liquid snippet and sections/header.liquid, focusing on the captured section output before the replace and split filters run. Compare the resulting value with the expected array element and determine whether section rendering preserves or changes the inserted string; done means the documented example behaves as expected or the limitation is clearly explained.