I noticed I cannot export product categories while exporting account.invoice records. I dig into the code a bit and found this:

<div t-name="Export.TreeItems"
    t-foreach="fields" t-as="field"
    t-att-data-id="field.id"
    class="o_export_tree_item"
    tabindex="-1"> <!-- tabindex make the div focusable -->
    <span t-if="field.children &amp;&amp; (field.id).split('/').length != 3"
          class="o_expand_parent fa fa-plus"/>
    <span class="o_tree_column"><t t-esc="field.string"/></span>
</div>

I removed this condition that check how many text block there is after splitting with "/" character, which is how many levels:

 && (field.id).split('/').length != 3

But still, it doesn't show any + signs after 3 levels :(

I suspect in the code something that would set field.children to false when getting to level 3, I don't know.

My next question will be how to make a module that change this. I want to allow drilling to 5 levels.