advisors.vanguard.com >> search for BND >> portfolio >> holdings details
which lists face and market values for its 17,194 holdings:

I clicked the "export full holdings," opened it in Excel, and was able to strike totals.
1) As of 3/31/2022:
Total face value: $296,344,367,296.89
Total market value: $301,952,316,721.49
Face value as percent of market value: 98.1%
2) A cursory quick Google search turned up a statement that according to Moody's, the long-term default rate for BBB/Baa-rated bonds--the lowest investment grade bonds--has been about 0.3%. So, less than that for Total Bond's average-AA Treasury-heavy portfolio, but let's say 0.3%
So far, facts.
3) I am going to interpret this naïvely as setting some rough floor on how bad things could possibly get.
The face value of the bonds in the fund is 98.1% of the current total market value. I believe that 99.7% of the bonds currently held in the fund will pay out their face value someday. (Usually this would be at their maturity date but in the case of mortgage-backed securities it could be earlier.) Therefore, the current holdings are sufficient to pay out 99.7% of 98.1% = 97.8% of the current value of the fund just by maturing... not including the coupon interest they will pay.
I am not as naïve (or dishonest) as to say the downside is limited to -2.2% because all of the bonds have various times to maturity, and every bond in a bond fund has a fluctuating market value subject to interest rates. And we know that BND has already fallen much farther than that. And bond funds sell their bonds before maturity, and by the time they sell them, they will have replaced them with new bonds, and so on and so on. And by the time they pay out, inflation will have reduced their real value.
Nevertheless, I think it is relevant to observe that, based on assumptions of low default rates for investment grade bonds, the face values of the bonds currently in Total Bond are adequate to pay out 97.8% of the current market value of the fund.
4) This is not a short-term prediction tool! With a 6.9-year maturity, the value of the fund could easily jink up and down by ±7% if the relevant interest rates jink up and down by 1%.
I am a buy-and-hold investor who intends to hold Total Bond for a time frame of its 6.9-year duration--or, let's say, the "4-10 years" Vanguard says "may" be appropriate. I would not use the total face value to judge that the fund is overvalued or undervalued, "expensive" or "cheap," or that it is a "good time" to buy/sell/short/leverage/whatever.
All I am saying that most of the dollar value of the fund is solidly supported by principal value inside the bonds. Dollar values that are locked and inaccessible, but almost certainly there.