In SE Android app v1.0.67, if you tap 'Add comment' on a post, then press the same button on a different post (in the same Q&A thread, without pressing 'Back' button) without cancelling the comment UI properly, the button on previous post will be gone, and it's impossible to add a comment on that post until you reopen the thread.
- Find a thread with at least 2 posts (e.g. 1 question & 1 answer)
- Press 'Add comment' on the question
- Press 'Add comment' on the answer. The button on the question will be gone
- Posting a comment or cancelling it properly (by clicking the button on the bottom left) will return the button on that post
(Click the image for its larger variant)