Should Text Format: "Full HTML" create a safe value?
The core text module does. This logic is driven by the text module's text_field_load(), an invocation of hook_field_load(). When the module loads text fields it looks up the filters used for the text format and sees if it's cacheable (i.e. if it saves to safe_value). 

In your case, the logic is being triggered by toggling the filtered_html / full_html formats. Because raw HTML is potentially unsafe, it's not stored in the save_value cache.
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