string registered with pll_register_string disappears

I’m trying to integrate Polylang into one of my plugins. When I register a custom string using pll_register_string() function I can see it show up in the Translations tab but then the string disappears from the list if I refresh the page. I can reproduce this easily by adding the following code in my theme, and then commenting out the pll_register_string() later to see if the string persists.

function test_polylang() {
	if ( function_exists( 'pll_register_string' ) ) {
		pll_register_string( 'test', 'test value', 'my-text' );
	}
}
add_action ( 'admin_init', 'test_polylang' );

Am I doing something wrong or have I misunderstood what the pll_register_string() function does? I am trying to replicate WPML string translations. Note: I already have a wpml config file but the strings I’m trying to register are added by the admin user.

Thanks,
Daniel

https://ww.wp.xz.cn/plugins/polylang/