@disorderlyf As a sighted person I usually read that capitalizing each word in the tag is preferred, and I prefer reading such.
The screen reader Orca seems to pronounce the underscores (as "line"), but otherwise the words are also separated.
The app suggesting lowercase tags is a known issue, https://github.com/mastodon/mastodon/issues/20267, it most likely suggests the first casing it has recorded unless it was later edited by the instance admins.