Hi Joseph, I'm using voice Microsoft Zira Desktop - English (United States) with theĀ Microsoft Speech API version 5 synthesiser.
I've just tried it with one of the other voices in SAPI5, Hazel, and the speech viewer window is also not closing with Alt+F4. Zera is my day-to-day voice and I'm sure Alt+F4 would close the Speech Viewer window before the update to NVDA 2019.3