2021a fails to create tabs containing axes

I have an app that runs perfectly well in Matlab 2020b. It includes a tab group of six tabs, 2 of which contain tables while the other 4 contain axes. Today I attempted to run the app in Matab 2021a. However, the run failed with an error message:

Only the two tabs containing tables were created, while the tabs having axes objects were not.

The creation code for each tab is

app.TabName = uitab(app.TabGroupResults);

I don't understand the (ungrammatical) error message or the source of the error, which presumably has to do with the parent/child relationship of the tab/axes objects.

Any help welcome

Gordon