Shelly1-PM power item doesn't update in sitemap

When switching off a shelly1-PM it correctly updates channel meter#currentWatts but the corresponding text item dosn’t update the value in the sitemap:

2020-06-11 12:53:02.738 [ome.event.ItemCommandEvent] - Item 'Shelly1pm1Schalter' received command OFF

2020-06-11 12:53:02.743 [nt.ItemStatePredictedEvent] - Shelly1pm1Schalter predicted to become OFF

2020-06-11 12:53:02.770 [vent.ItemStateChangedEvent] - Shelly1pm1Schalter changed from ON to OFF

2020-06-11 12:53:02.823 [vent.ItemStateChangedEvent] - Shelly1pm1Leistung changed from 59.5 W to 0.0 W

Does anybody know how to fix this?

In case of poor refresh with sitemap based UIs, first check openhab.log around load time for messages relating to sitemap file.