If I interpret this sequence diagram correctly, in case of invalid input, the Error message should end the activation of Sign Up Service. That is, just because a branch is the last one it shouldn’t be the one deactivating the participant, as activation doesn’t fall through branches (well, I think it shouldn’t) since they represent alternate flows of execution. In other words, if the valid input branch went first, it would end up activation of Sign Up Service and the error message would appear to be sent out of activation.
I imagine this is all due to a limitation of Mermaid, as I tried to deactivate SUS in the error message and got an error. I would report this to Mermaid but wanted to double check first, I also imagine someone else has already reported it.
The way I see it, the activation should end on the error message, and start again on the line dividing alt branches, to represent that on this branch, activation remains on since the GET /sign_up message.