How to level up Silver Kenshi to the maximum possible in Mortal Kombat X in Android
Solution 1:
This happens if you use a level up card to reach level 50, it overshoots. 50 is still the max level, so you not getting any more XP is the correct behavior.
Solution 2:
From this post, 50 is the max level for cards. This spreadsheet shows you the stats for all cards at minimum level (level 1, fusion 0) and at maximum level (level 50, fusion VII).