You will have to verify the charging rate once voltage at battery terminal reaches 13.8 Volts, sometimes during trickle charging if charging rate at last stage decrease to 1-2 Amperes then UPS continues to charge the battery and voltage on battery terminal does not touches the cut off Voltage like if its set to 14.2 Volts. In-order to resolve the issue, you will have to increase the last stage charging so that it is able to push the Voltage up to 14.2 Volts or on what so ever your cut off voltage is set.
You can assume the example of a vehicle climbing up a mountain with a Force and Speed. If surface is slippery then even with more force and less speed wont produce the desired results, you tires will keep on slipping and you wonāt experience any gain. You are burning your tires, so here you just evaporating your distilled water content in electrolyte.
You have to reach the hill top slowly but not so slow that you donāt climb up. Setting it to 3-4 amperes on last stage will resolve the issue.