Hello,
Thank you for contacting Rank Math and bringing your concern to our attention. I’m sorry for the delay and for any inconvenience this issue may have caused you.
For initial troubleshooting, please confirm if you have already set the Primary Taxonomy in Rank Math > Titles & Meta > [Post type]:
https://rankmath.com/kb/how-to-choose-a-primary-category/
Also, select the child category as the Primary Category for those single post types

Then, ensure that this option is enabled in Rank Math > General Settings > Breadcrumbs

And this is disabled as well

Let us know how this goes.
Thank you.
Hello,
Thanks a lot, the first step was not complete but now it’s good with your help!
Thanks,
Hello,
We are super happy that this resolved your issue. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.