DM visible by the OP without mention to it - is it intended?

I noticed that when replying to a message and switching its privacy in DM and after removing the mention to the OP, the message is still visible by the OP (even if not mentioned and it will receive a notification).

Some tests:

Mastodon to Mastodon -> DM not visible if no mentions

Mastodon to Pleroma -> DM not visible if no mentions

Pleroma to Mastodon -> DM visible even with no mentions

Pleroma to Pleroma -> DM visible even with no mentions