Is the UML sequence diagram with two activations instead of one correct?

The following sequence diagram is rendered by Sparx Enterprise Architect. Is the start of the new activation (blue) correct? Shouldn't it rather continue with the existing activation on the Object2?

Please note that the flow starts with the asynchronous (red) call.

If the diagram is correct, why the new activation is not started in the following (almost identical) case?

UML leaves you full flexibility about the execution specifications. So it is indeed a choice whether the blue message is in a separate activation or within the same activation.

EA makes tries to make an educated guess. It is not clear to me why in the second case it continues and in the first not, but it may be related to some heuristics.

Regardless of what EA proposes, you can however override it, by right clicking on the relevant message to get the contextual menu. You may then chose Extend Source Activation Down to make the previous activation continue.