If you open TalkBack’s Settings window and open the Verbosity option, you can enable or disable a setting called “Speak letters with examples.” If disabled, letters shouldn’t be spelled phonetically after each actual letter upon navigating. However, TalkBack keeps spelling letters phonetically when navigating letter by letter, even when this setting is disabled.
To duplicate, disable this setting if it’s not, and move to the Home screen or the App screen. Try navigating around letter by letter by setting the Reading control to Letters. You should hear, a. alpha, i. India, m. Mike, l. Lima, etc.