I would add one more possible cause to the list

Also check ((any of  ) it's grand-parent Structure group properties:

if it is set to Publishable=false.

Although it's seems not logical place to look for but I have seen this behavior.

I would add one more possible cause to the list

Also check (any of) it's grand-parent Structure group properties:

if it is set to Publishable=false.

Although it doesn't seem a logical place to look for, but I have seen this behavior before.