If you follow a channel you will only be notified if the team channel is @ mentioned in the post. An incoming email will not contain a channel @ mention. You could create a Flow for a new post in a channel to send you an email, there is a template in Flow for that.
If you follow a channel, you will get notified according to the settings! You can set to get a banner and email for followed channel conversations and also separate settings for @mentioned channels! What you experience is that if you are active in the client when recieving messages you should get a banner but no emails will be sent out! This is per design! Currently the status is not switched immediately either which can result in you being away some minutes and status is not yet set as away and no mails will be sent
You only get e-mail if you are not active in Teams. Regardless of the setting. E-mail coming into a channel used to and may still have issues triggering notification events, but you must be in away status and meeting the "frequency" setting you have for your sending options before e-mail will be sent saying you are missing messages/activity in Teams.