※This event is not a Bug, it is "Because the required cumulative EXP has not been reached" instead.
Previously, there was an issue wherein Trophies and Badges leveled up even if the EXP from the materials did not reach the required EXP to level up.
※The aforementioned bug has been corrected in the update implemented on December 24, 2019, however, the level that has been already upgraded by the issue has not been adjusted.
Therefore, you may see that the EXP gauge is full, however, in reality, the accumulated EXP required for the current level is insufficient.
Current level: 70
Real EXP: Equivalent to level 69
As per above-menioned, you need to make up for "One Level" of EXP
Therefore, it is not possible to level up to "Level 71" without accumulating EXP from level 69 to 70 (required EXP: 2400) and level 70 to 71 (required EXP: 2040)