Context. In sunspot umbrae, the core of some chromospheric lines exhibits periodic brightness enhancements known as umbral flashes. The consensus is that they are produced by the upward propagation of shock waves. This view has recently been challenged by the detection of downflowing umbral flashes and the confirmation of a resonant cavity above sunspots. Aims. We aim to determine the propagating or standing nature of the waves in the low umbral chromosphere and confirm or refute the existence of downflowing umbral flashes. Methods. Spectroscopic temporal series of Ca II 8542 Å, Ca II H, and Hα in a sunspot were acquired with the Swedish Solar Telescope. The Hα velocity was inferred using bisectors. Simultaneous inversions of the Ca II 8542 Å line and the Ca II H core were performed using the code NICOLE. The nature of the oscillations were determined and insights into the resonant oscillatory pattern were gained by analyzing the phase shift between the velocity signals and examining the temporal evolution. Results. Propagating waves in the low chromosphere are more common in regions with frequent umbral flashes, where the transition region is shifted upward, making resonant cavity signatures less noticeable. In contrast, areas with fewer umbral flashes show velocity fluctuations that align with standing oscillations. Evidence suggests dynamic changes in the location of velocity-resonant nodes due to variations in the transition region height. Downflowing profiles appear at the onset of some umbral flashes, but upflowing motion dominates during most of the flash. These downflowing flashes are more common in standing umbral flashes. Conclusions. We confirm the existence of a chromospheric resonant cavity above sunspot umbrae. It is produced by wave reflections at the transition region. The oscillatory pattern depends on the transition region height, which exhibits spatial and temporal variations due to the impact of the waves.