[Sitemap] Entry not added to robots.txt

[rcnascimento]

Describe the bug
The sitemap is enabled but the link is not added after my custom robots.txt content.

To Reproduce
Steps to reproduce the behavior:

1. Make sure the Sitemap is enabled and working (menu "Sitemap Settings");
2. Save a custom robots.txt via plugin (menu "General Settings", tab "Edit robots.txt");
3. Open the generated robots.txt file in browser;
4. The line with sitemap reference is not there.

Expected behavior
The plugin must auto add the sitemap index reference to the robots.txt. At the end of generated file.

[rcnascimento]

I have changed this hook callback priority to 10 (the usual) and now the line appears on my robots.txt:

seo-by-rank-math/includes/modules/sitemap/class-sitemap-index.php

Line 30 in 13a1ce8

$this->filter( 'robots_txt', 'add_sitemap_directive', 0, 2 );