Heartbeat interval not working correctly

Hi. I have used this plugin to change the interval for wordpress backend heartbeat, and post editor to 300 seconds for each. However, when using Chrome developer tools to monitor the ajax requests from the site (for example when editing a post), I can see that the heartbeat interval is actually operating at 120 seconds. This seems to be different to the default 15s for WordPress at least but I can’t see why it is operating at 120s and not 300s?!

Are there any widely used plugins or settings that might be conflicting with this? Or any other good way to verify whether the set 300s interval is really working?