Disappearance:
Fix:
1. The first point is to ensure that the number of cells is correct ;
2. The second point is a small detail. You need to add a "|" after the c here :
\multicolumn{3}{c|}
Of course, if the left side is less, then add the left side; or it is easiest to just add both the left and right sides:
\multicolumn{3}{|c|}
Repair effect: