My apologies Alex, I have raised an issue where there is no issue....
I know why, still silly of me though for not trying a different category (see image).
I used the Information category (the first category i set up in the forum a while ago) to test container at YES and went straight to New Discussion. The category appeared in the select a category bar automatically, without any need to use the dropdown or select, I assume now because it is ID 1 and I have categories set to run in ID order.
I tried to make a post, clicked submit and nothing happened.
I tried different access (all, some none) permutations with the Information category, checking the dropdown, but because the Information category is ID 1 and auto appeared on the category bar it was also highlighted in blue when it appeared in the dropdown, so I didn't notice it was a faded colour when container was set to YES during my tests.
Therefore I made my post about the 'issue'.
When you replied and I went back to test with a new 'Public Area/General' category/sub-category, I was testing with no user access on category selection because I did not want anyone to be able to post and having 'tested' on the Information category before, this seemed to be the only way of achieving the outcome from your first reply.
Sorry again for wasting your time Alex, soooo embarrassing...............