The spike is just a temporary amplification of the normal "Message x is not defined" warnings which have existing tasks filed already.

They are amplified after deployment because deployment rebuilds localisation cache which triggers a revalidation of ResourceLoader message blobs. This is intentional and ensures that updates messages are actually deployed. As part of this, it will also walk over messages that turn out not to exist. These warnings are mostly dormant at other times because messages are strongly cached.

Closing this task because it essentially just says "Errors happen when broken code is triggered." – where the relevant code here is code triggered by deployment. (Not this deployment in particular, but any.)

Example of a task for the broken code itself:
T118820: MessageBlobStore::fetchMessage failed to find: 'multimediaviewer-option-header-viewer' (and other multimediaviewer keys)