I use the default theme, no modifications whatsoever to the theme.
Yet, created a now vault and recreated the note. The previously hidden lines are now permanently visible, though there is no folding triangle (there is, for the # header e.g.).
However, it seems to be connected to the tasks plugin. I installed the plugin into the fresh vault and restarted Obsidian and immediately had the behavior described in the original post. Disabling the plugin reverted it.
Hmm… I don’t use that plugin regularly, only when testing stuff, but if it’s the case you should probably consider posting an issue on the github page of the plugin. Or check if it’s already reported.