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If YTM drops before a coupon is paid, that means you will reinvest your coupon (which is fixed $) at a lower interest rate than at inception (when you first bought the bond).
Therefore, you’ll get a lower yield from reinvesting your coupon ==> reinvestment risk increases.
Since the bond is held to maturity, this means that there is less/no price risk (i.e. making a loss if the bond is sold before maturity, where sale price may be lower than purchase price if YTM has increased).
Both price risk and investment risks offset each other partially. The shorter the investment horizon, the smaller the coupon reinvestment risk, but the bigger the market price risk, and vice versa.