When adding a new picklist entry, the new entry does not immediatly appear in a referenced PowerApps lookup control. Sometimes it can take several days to appear. E.g. A new entry was added to a picklist on 23 January and it is still not available in the lookup control.
If the picklists is added after App creation, then it may not be available in the current App.
Please take a try to remove the entity added from the Common Data Service, then add it again under the Add data source pane.
View->Data Source->Data Source list
After that, check to see if the newly added field could be loaded in PowerApps.
It is not a new picklist but a new entry in an existing picklist used in an existing app.
The refreshing of picklist entries has been a known issue for at least a year. Sometimes the new entry is available in a few hours or days, but recently new entries are taking much longer to appear, and my latest case, a few weeks. Last time I submitted a problem report to MS it was fixed the next day.
Does anyone know why it takes so long for MS servers to refresh the picklist entries for CDS datasets?
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Features releasing from April 2020 through September 2020