I have a plugin with settings (via Plugin:SetSetting) that specifically disable the plugin during playtesting. It appears this, along with other settings for my plugin, keeps getting reset whenever I run a solo test.
This plugin should not be showing up in the first place because I have a setting (“detectSoloMode”) set to true. But other than this, there is a lot of evidence that none of my plugin settings are being transfered into solo mode.