I do mini test – old post, created before WP 4.4 and Lightbox Plus Colorbox work OK.
Code:
<a href=".../image.png" rel="lightbox[1234]" title="TITLE"><img class="aligncenter wp-image-1234 size-full" src=".../image.png" alt="ALT" width="1000" height="800" srcset=".../image-650x415.png 650w, .../image.png 1000w" sizes="(max-width: 1000px) 100vw, 1000px" /></a>
Next I edit that post, remove image and insert once again. Save and Lightbox Plus Colorbox not work at this img:
<a href=".../image.png" rel="attachment wp-att-1234" title="TITLE"><img class="aligncenter size-full wp-image-1234" src=".../image.png" alt="ALT" width="1000" height="800" srcset=".../image-650x415.png 650w, .../image.png 1000w" sizes="(max-width: 1000px) 100vw, 1000px" /></a>
Work: <a… rel=”lightbox[1234]”
Not work: <a… rel=”attachment wp-att-1234″