Problem when merging object with Probuilder

When I merge two objets in Probuilder I get a new object whose I cannot grab/move its vertices, edges or faces (like it is not Probuilderised). When I try to pick/move an edge I get the following message in the console:

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KeyNotFoundException: The given key was not present in the dictionary. System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) (at :0) UnityEngine.ProBuilder.ProBuilderMesh.CacheSelection () (at Library/PackageCache/com.unity.probuilder@4.2.3/Runtime/Core/ProBuilderMeshSelection.cs:112) UnityEngine.ProBuilder.ProBuilderMesh.get_selectedSharedVerticesCount () (at Library/PackageCache/com.unity.probuilder@4.2.3/Runtime/Core/ProBuilderMeshSelection.cs:64) UnityEditor.ProBuilder.MeshSelection.RecalculateSelectedComponentCounts () (at Library/PackageCache/com.unity.probuilder@4.2.3/Editor/EditorCore/MeshSelection.cs:268)

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I have been using Probuilder for one year now and I did not have this problem before. I recently updated Unity to 2019.3.5f1 (but the issue did not happen directly after the update) and I use the latest Probuilder 4.2.3. It happens also to older projects.

Here you can see a prototype tree before and after merging.

Anyone has the same issue? Can you help me or direct me to a corresponding thread?