Hi gulliver,
The custom title function doesn’t cover the titles for single posts or pages. In order to control that, you would either have to add some custom code or use a plugin like All in One SEO: https://wordpress.org/plugins/all-in-one-seo-pack/
Thanks.
On other installs I’ve manually coded various conditionals – but was trying to avoid it this time.
(Still looking for that rarity… the ‘sensible’ theme which looks good and is well-coded with genuinely useful features and none of the silly stuff. Hemingway is the nearest I’ve yet found… so I was being optimistic in hoping for a little more which’ll made a good theme great.) 😉