There are a number of things that can cause this, most are simple to check and easy to fix. Step one is always to check the source code - that is, don't assume that what Google is showing is exactly what is on the page itself. View the live html of the page you want to check.

Some themes output content from your settings and your plugin. Some themes have no support for the WordPress title hook. Yoast's default has always been

%%title%% %%page%% %%sep%% %%sitename%% 

which would produce "Your Chosen Title | Sitename" or "Your Chosen Title P2 | Sitename" as examples. But that is not set in stone and not found in every theme. WP gets this information from Settings > General : Site Title and from from Settings > Permalinks.

Yoast uses your custom edited titles just above where you can customize the page description. It may be necessary to look through your Yoast plugin settings to ensure that "Force Rewrite Titles" has been set in Yoast's General settings.

You can read more at their support pages. This page is a good place to start: https://kb.yoast.com/kb/the-seo-title-output-for-the-plugin-doesnt-work-as-expected/