You've a mis-understanding of how funds like FDLXX are managed. In a T-BILL only fund even a decrease in past asset value doesn't matter because they are by law managed to 1$ of net asset value. That is they most hold 1$ of _current_ asset value for every 1$ deposited _at all times_.

The only situation where the value of the fund can be less than what you put in is the collapse of US currency, which savings account insurance can not protect against either.