This is the output of go run main.go:

❯ go run main.go
INFO[0000] calling valuer
FATA[0000] can't get tests: can't scan into dest[1]: json: cannot unmarshal string into Go struct field Test.timestamp of type timestamppb.Timestamp
exit status 1

It seems that the Scanner interface is not being called when scanning a timestamp field into a timestamppb.Timestamp field.

This leads to pgx to try to unmarshal timestamp using the json package, which fails. It should use the protojson package instead.