Hello,
Thank you for getting in touch with us. We apologise for any inconvenience this might have caused.
If you use different heading blocks (say H2 and H3), lower-level heading blocks will be indented to show the structure of your content. For instance, if you start with an H2 Heading block and then add an H3 to H5 below it, these will get incrementally nested and added as sub-headings, as shown below:

In your article, the first heading shown in the Table of Contents has the H3 heading block, so Rank Math will automatically add H4, H5 heading blocks below it and ignore the heading blocks with H1 and H2 though they appear later in the article.
This is for proper structuring of your content.
To resolve this issue, you need to start your article with an H2 heading block and the other heading blocks will be nested below automatically in the Table of Contents.
Please follow this guide as a reference: https://rankmath.com/kb/table-of-contents-block/
Hope this helps, and please do not hesitate to let us know if you need our assistance with anything else.
Thank you for choosing Rank Math.
HI Adetayo,
Thanks for sharing your knowledge, this is extremely helpful!
Have a good day.
Best,
Luke
Hello again
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.