I just spoke with customer service and they ackowledged the issue (battery drain after updating to 4.4.9344 for Kobo Glo HD).
I know it is not affecting everybody - my wife has the exact same model as mine with the same version of the software and her device does not suffer this issue.
The complete response from Kobo was (in French)
Merci d'avoir contacté le service à la clientèle Rakuten Kobo. C'était un plaisir de vous aider aujourd'hui.
Vous nous avez contactés aujourd'hui car après la mise a jour la liseuse ne s'étteint pas et la duration de la batterie est raccourci.
Comme indiqué au téléphone, ceci est un problème connue de kobo et une mise à jours est prévu pour résoudre ce type de problème.
Le lancement de la mise a jour est prévu durant le moi de juillet.
Which translates to: you contacted us because after the update, the backlight does not turn off during sleep and the battery life is reduced; as discussed on the phone, this is a known issue from kobo and an update will address this issue. The update is scheduled for July.
*** EDIT
Update on this, downgrading, performing a full factory reset and upgrading again fixed this issue for me - the screen now correctly dims when the device goes to sleep.