Update: I have been able to reproduce and fix this issue in update 3.1.2, which is now live.

Original: Thanks for your review and feedback. On my end the button is still showing on all browsers - it is possible that YouTube is testing out a new UI with some users which is causing the extension to not work as expected for you anymore. If you would be so kind to open a bug report on the project's GitHub page (https://github.com/NikkelM/Random-YouTube-Video/issues) and include a screenshot of a channel and video page, and we can work together to find the issue.