Hello,
Thanks for contacting us, and sorry for any inconvenience that might have been caused due to that. We deeply apologize for the delay in response.
Firstly, this might already be enabled on your website, but please ensure that the Show Category(s) option is enabled under Rank Math > General Settings > Breadcrumbs:

Then, edit the product page and set the child category as the Primary Category, and see if that gives you the desired output. Here’s how you can set the Primary Category: https://rankmath.com/kb/how-to-choose-a-primary-category/
Let us know how that goes. Looking forward to helping you.
Thank you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.