Moderation Action PubSub topic only works on own Channel?

Yes it would appear that having moved the topic into official support, it was also updated to be more in line with current methodology and scoping.

To monitor all moderator events you need to use the new “channel topic” with an oAuth provided by the broadcaster.

It seems that it is no longer supported (technically never was) for you use get this data when authenticating as a moderator.

Changes/it being documented originally covered here Upcoming OAuth scope requirements for PubSub