This is the expected behavior at the moment. It may change in future.
Before the 8.8 update in Telegram on iOS, photos taken in the app were not saved to the device gallery.
As they answered me today, it was a bug, but it looked like a completely logical scenario, given that the "do not save edited photos" switch was in the settings.
It was very convenient to send to the friends photos which did not required saving to the gallery
For example, a photo answers the question "where are you?" much better, while this photo will never be needed again, so there is no need to store this photo on the device.
If I needed to save a photo, I took it through my native camera, using the greater functionality of the camera application.
I propose to return the ability to disable saving photos taken in dialogs to the photo application. This can be done by an optional setting in settings >data