The 16c7594 commit (found by bisecting) revealeda bug in ascii-plot.
Attached is a fix.

To test, consider this table:
| 1 |
| 2 |
| 3 |

The cursor being in the table, type C-c " a


Result with the bug:
| 1 |   |
| 2 |   |
| 3 |   |
#+TBLFM:  2='(orgtbl-ascii-draw $1 1 3 12)


Result with the fix:
| 1 |              |
| 2 | WWWWWW       |
| 3 | WWWWWWWWWWWW |
#+TBLFM: $2='(orgtbl-ascii-draw $1 1 3 12)