I do not own Bond Funds but I do have about 75 individual muni bonds mostly rated AA. A Schwab algorithm prices these daily without regard to market sales which are very infrequent. Nevertheless, my bonds on average suffered a 2.5% drop in value yesterday when the DOW closed down 2300 points. Similarly, last week when the 10-year dropped almost 50 basis points in a single day I did not see the huge price increase that one would expect.
I then spent some time looking for actual trades that occurred recently. Only a few bonds had actual trades within the past week but they were at a much higher price than the algorithm. During the stable times the algorithm is fairly close to predicting value but now it appears way off.