Do you use other Ecowitt devices besides the WH51 soil moisture sensor where you can check the data report interval?
I assume that the change in soil moisture can be strongly influenced by external factors, e.g. where the sensor is installed and how deep, when it last rained, is it windy or not, etc.
Even with my WH51 sensors, it sometimes takes several hours before a new value is reported. However, this has nothing to do with the sensor or the data transmission interval, but with the soil moisture itself.
The moisture level of one of my sensors didn’t change for about 19 hours either. But the temperature interval on my WS90 is sometimes shorter than 1 minute:
