You're dang right I'm sure and there's nothing novel about them. Senior/junior, etc. only denotes seniority in rights. Just because you have an ownership interest, i.e. - SPS, JPS, Commons......that doesn't mean the ownership interest has any value (in the case of negative net worth).
You seem to be confused by the SPS cumulative dividend feature.
The GOV has called them equity and the money "sent" to F and F is called an investment not a loan I imagine the GOV books reflect this
At same time - normally Preferred Stock are issued without an expiration date - for last 20 years or so (in prior years most JPS had maturity dates).
I do not think the SP have a maturity date but --- but ---- I do believe the GOV has the right to demand payment as if they matured - a put on F and F (this put might be after x years like 10 years OR OR when F and F have X in capital OR OR before F and F can again return to ---- ====== . Thus while an investment the GOV has a right to demand they be redeemed !!! So that cost to F and F - about 200B in cash sits on them as if it were debt and maybe more like DEBT that is matured --- but redemption is delayed while dividends are paid?