It could be, it looks like there was some versions misalignment:

The maps syntax is only supported starting from v3.0.0. The "proto2" in the doc is referring to the syntax version, not protobuf release version. v3.0.0 supports both proto2 syntax and proto3 syntax while v2.6.1 only supports proto2 syntax. For all users, it's recommended to use v3.0.0-beta-1 instead of v2.6.1. https://stackoverflow.com/questions/50241452/using-maps-in-p...