HTML code editors removes code!
I suppose that your issue is caused by fact that TinyMCE editor removes some tags.
You can try to override editor config: https://getpublii.com/dev/how-to-add-custom-config-or-js-for-post-editor/
You should use something like this:
Do you appreciate the support you've received today? If so, consider donating to the Publii team by clicking here; we'll be sure to use your donation to make Publii even better!