I remembered later that I had the same issue on the iPad Pro 12.9” (2nd Gen) which I no longer own. So I ended up submitting a bug report, because the problem is not restricted to facebook. I’ve had similar issues in a couple of forums as well, in fact anywhere which uses popup emoticons and at this stage it seems to only be Safari, I alos use Firefox and the duckduckgo browser and both of those behave as they should.
Bug reports here
https://www.apple.com/feedback/