Why are my data plane proxies on a different version after an update?

1 
% istioctl version
client version: 1.15.0
control plane version: 1.15.1
data plane version: 1.12.1 (14 proxies), 1.15.1 (20 proxies)

I just did an update, but my data plane version is still on 1.12.1. How can I update that?