I have discovered that all the system text on my site that is wrapped in the <legend>System Text</legend> tag is not visible on my website:

this is what the text in tag looks like on the default @joomla user profile settings page:

this is how this text looks in the default Cassiopeia Joomla template:

all other text in any other tags is visible on my website.

This is a problem with the @gantry template, not Joomla.

Joomla 5.2, Gantry 5.5.19