If you were satisfied with your Istibraa (i.e. you were sure that the urine drops had stopped) and you performed your Salaah with the tissue on, then your Salaah was valid. When making Istibra, the urine drops that emerge and get onto the tissue (prior to making Wudhu) can be overlooked as long as they are less than the size of one Dirham (2.5 to 3cm in diameter). Although Salaah will be valid if performed without removing the tissue (that has absorbed such a minute amount of urine), it would be advisable to remove it before performing Salaah even though one is uncertain as to whether urine drops got onto the tissue or not. However, Salaahs that were performed in the past by leaving the tissue (that had such a minute amount of urine drops) do not have to be repeated.
If some urine drops also got onto the private part or its surrounds, it would be Makruh (disliked) to perform Salaah with such minor impurities on one’s clothes or body; therefore, it would be better for one to wash off the impurity even though the urine drops are less than the size of one Dirham. And if the urine drops are more than this amount, then one should repeat the Salaah that is performed in this impure state, whether such a large amount of urine was on the body, clothes or on the tissue that was not removed.