An incision that opens after it seemed to be closed is a known problem, and it has a name.
Sometimes an incision breaks open, either along its whole length or in part. What happens next is something patients are rarely told ahead of time: the surgeon may choose not to close it again with stitches. The wound then heals from the inside out. A dressing absorbs drainage and keeps the skin from closing over the wound before the tissue underneath fills in.
That explains something that can otherwise look like neglect. Leaving a wound open can be the treatment, not a lack of treatment. The dressing is doing a job — holding the surface open while the deeper tissue fills in.
Contact your provider if you notice: more redness, pain, swelling, or bleeding at the wound; the wound getting larger or deeper, or looking dried out or dark; drainage that increases or becomes thick, tan, green, or yellow, or smells bad; or a temperature of 100.5°F (38°C) or higher. That temperature applies to open surgical wounds specifically.
When a surgical wound comes apart, it’s called wound dehiscence. Wound Wizard also has a page on wounds that keep reopening.