I have had this same problem, but it happens quite often. I have encountered it on a iPhone 6, 7plus, and iPhone 8, and the 12.p iPad Pro.
VoiceOver May on Occasions Become Unresponsive and Need to be Restarted
This behavior itself is not new to iOS in iOS 11, but we have detected what we believe to be a slight increase in the frequency of voiceOver becoming unresponsive and needing to be restarted to resume normal working behavior. In particular, it seems more common that spoken output from VoiceOver will stop, but the other VoiceOver sound effects continue.