Multi-line comment highlighting breaks after row deletion

How to reproduce

Open a .c file with 4 lines:

int a;
int b;
int c;
int d;

1. Move cursor to start of line 2 (int b;), press Backspace — merges it into line 1
2. Move cursor to end of line 2 (int c;), type /*
3. Line 3 (int d;) should turn comment-colored but stays normal

For a control test, open the same file fresh (skip step 1), type /* at the end of any line — the line below immediately turns comment-colored.