I am having the same issue described in this question when using the AGU template on overleaf.
In the agujournal2019.cls file, however, the \bibliographystyle is already set to apacite (line 1091) as suggested in the question mentioned.
Anybody has this issue or an idea of how to deal with this? I have created and shared a project on overleaf to show the issue and share the .cls file in question.