What is the streaming loudness preview?
It plays your master at the level each platform will play it, so the turn-down is something the client hears.
Pick a platform on the client review page and playback moves to the gain that platform applies after its own loudness normalization. Spotify, Apple Music, YouTube, Tidal, Amazon Music, TikTok and Bandcamp are all there, each with its published target.
The gain is arithmetic: the platform's target minus your master's measured loudness. Some platforms only turn things down, some also turn quiet material up, and Bandcamp does neither. The readout names the platform, the number and the direction, so nobody has to take it on faith.
What it does not do is decide anything for you. A target is a reference point, not a ceiling, and how loud a record should be is part of what you were hired to judge. The preview exists so that judgement is made with the platform in the room, and so a client hears the same turn-down their listeners will instead of discovering it after release.
For Spotify the limiter behind it follows their published behaviour, which reacts in 5 ms, so the fastest peaks pass above the ceiling. You are hearing that, not a fault in the player.
Not covered here? support@soneam.com.