DS + Page Manager: Implementing preprocess hook breaks output

I'm not sure yet if this is a page manager or a display suite issue, I didn't try yet to dig in the code.

Combining display suite with page manager, and implementing a targeted preprocess hook, breaks the output and generates broken html:

Steps to reproduce:

1. Install D8.3.3-dev site.
2. Install ds-8.x-3.0-rc1 (also with current dev)
3. Install page_manager-8.x-4.0-beta2 (also with current dev) and page_manager_ui
4. Enable the "node view" page (admin/structure/page_manager) and add a variant of type "Block page". Add the default contexts. On the variant's "Content" add only "Entity view (Content)"
5. Visit Manage display for a content type (admin/structure/types/manage/article/display/default)
6. Use Select a layout: Display Suite - One column layout
7. Generate custom module my_module
8. Implement hook: function my_module_preprocess_ds_1col__node_article_default(&$variables) { } (just leave the function empty
9. Create a test node and visit the node/NID page. You will get the broken output.

As stated above, I'm not sure yet if this is a page manager or a display suite issue.
If page manager gets uninstalled, the output (with or without DS's layout) is as expected.
Also if DS gets uninstalled and page manager's variant is enabled, again the output is as expected. If another layout is used (not one of those provided by DS), again the output is as expected. This last point makes me think that it's possibly a DS issue.

I'll come back with further investigation, but if anyone has any idea, please drop it :)