drupal-render-placeholder instead of webform block

This is the potential bug report either of Webform, Block Token or Drupal core, I'm not sure yet. I've already spent a while of debugging this, but without success.

Basically I've couple of webforms with simple fields, lets call it the main webform, and the other webforms. These webforms are exposed as token using Block Token module. In the main webform I've created Computed token field with value:

1:[block_token:webform:myform1];2:[block_token:webform:myform2];3:[block_token:block_content:custom]

The problem is that when I'm viewing my main webform, the content of two webform blocks aren't rendered, although my custom block is rendered.

On the page it-self the output is empty:

1:;2:;3:<div id="block-test" ...><h2>Test</h2>...</div>

But when dumping the element in template_preprocess_form_element I see two placeholders for webforms, and properly rendered custom block:

string '1:<drupal-render-placeholder callback="Drupal\block\BlockViewBuilder::lazyBuilder" arguments="0=myform1&amp;1=full&amp;2" token="p8BOEzhVedaNVMwuFKbSbXYvHJtJev-99V8a_12Hv4A"></drupal-render-placeholder>;2:<drupal-render-placeholder callback="Drupal\block\BlockViewBuilder::lazyBuilder" arguments="0=myform2&amp;1=full&amp;2" token="CLE9WQ_qWlsCNZws4dp3bfLQFegyeEcEOg9tM-jrNwI"></drupal-render-placeholder>;3:

<!-- THEME DEBUG -->
<!-- BEGIN OUTPUT from 'core/themes/classy/templates/block/block.html.twig' -->
<div id="block-test" class="block block-block-content block-block-content76289379-5e8a-4435-84ab-cc9f26eec38d">
  
      <h2>Test</h2>
'... (length=1707)

And this is what is rendered in template_preprocess_form_element for (string)$variables['children'] (with another test with one webform):

1::2

<!-- THEME DEBUG -->
<!-- THEME HOOK: 'input__hidden' -->
<!-- FILE NAME SUGGESTIONS:
   * input--hidden.html.twig
   x input.html.twig
-->
<!-- BEGIN OUTPUT from 'themes/custom/one_manchester/templates/form/input.html.twig' -->
<input data-drupal-selector="edit-myform1" type="hidden" name="myform1" value="1:&lt;drupal-render-placeholder callback=&quot;Drupal\block\BlockViewBuilder::lazyBuilder&quot; arguments=&quot;0=myform1&amp;amp;1=full&amp;amp;2&quot; token=&quot;p8BOEzhVedaNVMwuFKbSbXYvHJtJev-99V8a_12Hv4A&quot;&gt;&lt;/drupal-render-placeholder&gt;:2" />


<!-- END OUTPUT from 'themes/custom/one_manchester/templates/form/input.html.twig' -->

I've read that normally you would need to use renderRoot() instead of render() to render these placeholders correctly, I've found few places in Webform which had render() and replaced them with renderRoot(), but it didn't help. The same happens on default Drupal theme. I've tried to reproduce the problem on the clean Drupal 8, and then I couldn't.

My steps on the clean Drupal were:

    drush qd
    cd *drupal*/drupal
    drush -y en webform_ui block_token
    drush cr

Configuration steps (I hope they're in the right order):

1. Expose any block via block_token, e.g. go to /admin/structure/block/manage/bartik_powered and tick 'Create the token for this block'.
2. Create Test form and Save at /admin/structure/webform/add
3. 'Add element', 'Computed token', Title: Test, Computed value: Foo:[block_token:system:bartik_powered]
4. Create Test form 2 and Save at /admin/structure/webform/add
5. View webform, the block is rendered correctly.
6. Go to: /admin/structure/block, click 'Place block' under 'Content', add Webform/Webform, then Place block, type test_form in Webform and select previously created test form, and select 'Create the token for this block'.
7. Add another 'Computed token' (Test 2), with [block_token:webform:webform] as a value.
8. Check whether webform tokens are rendered correctly on the webform view.

However as mentioned above steps worked fine on the clean installation, not on another site, so I'm not sure what's the problem, there could be some extra filter enabled, or some other missing thing.

Potentially related issue: #953034: [meta] Themes improperly check renderable arrays when determining visibility

Related question: How to replace drupal-render-placeholder manually?