The balance sheet value of these securities does not change when their market price changes. The amount by which the price falls below the purchase level is an unrealized loss. It is disclosed in the financial statements, but it does not change the value of the asset itself.
In general, if a bank sells part of its HTM securities, it must reclassify the whole portfolio as available-for-sale (AFS) and mark it to market. For this reason, selling is a hard way to adjust a position in this category.