The time interval properties of a layer used to configure the time slider are not supported in a web map. If the web map is shared with the time slider configured this way, the Number of Steps option is applied instead.
Under Map, on the Time tab, in the Step group, change the option to either Number of Steps or Step Interval.
The Time Interval properties for a layer are configured on the Time tab of the Layer Properties dialog box. When a map contains one or more layers with these properties, the map's time slider in ArcGIS Pro can be configured to reference them. This layer-based time slider configuration option is not available in a web map and cannot be included during the sharing process.