Asking this here because I think this is very likely to be a general behaviour.
Some months ago I wrote an answer to a question in Software Engineering that got very bad feedback (two negative votes and one delete) which then made me get 99 of reputation.
Now, I see this in the page
Highly active question. Earn 10 reputation in order to answer this question. The reputation requirement helps protect this question from spam and non-answer activity.
Understand that I was awarded 100 in reputation in that specific network due to having good reputation in other communities. Still, the description in the Highly active question page also mentions
Protecting a question prevents answers from being added by anonymous and very low-reputation users, generally users with under 10 reputation.
and I don't have under 10 reputation. So, either it should be clearer that "Highly active question" blockage can also apply to users with more than 10 of reputation OR give such users the permission to do so.
According to Ollie's answer it's intentional that such users don't have the respective privilege. So, this question is then only proposing to add the content from Ollie's answer (see right after) in the protected question's page.
The +100 reputation Association Bonus is ignored in such cases. From the post What is the Association Bonus and how does it work:
In other words, a user with the association bonus and no other rep changes can't answer protected questions.
On per-site metas, you can answer protected questions with the association bonus reputation, provided your corresponding main site reputation doesn't fall below 10.