I haven’t added any snippet regarding the comment form however, I have added a snippet to collect emails (custom code). Further, I also added some CSS to change the color of SUBMIT button.
I removed both the above and each snippet in the plugin. Then, cleared the cache and tested the site in incognito mode and on my phone too.
Very odd, i think Fernandos train of thought is most probably correct and another plugin is breaking it. If you can’t find the conflict then treat it with this CSS: