I'm seeing the same thing. I get the Quote button if the text is all of the same type (i.e. either all normal or all bold), but as soon as there's a mix I don't see the Quote button. I'm using the latest canary build of Edge. Have tested it in the latest stable build of Edge as well with the same result.
@tmomas - Yes, the forum was recently updated. I hesitate to consider a downgrade of the forum software due the potential for database migration issues which may be introduced. I would prefer to go forward if possible but, unless there was a specific fix for this available, I'm not sure this will help right now.
One thing on my "to do" list here is an item to take the site off-line for about 15-30mins and do an application rebuild rather than an upgrade. This action, among other things, will re-build all assets (js, css, etc.). I believe it was recommended by the Discourse folks a few years ago to be done a few times a year (quarterly?). The OS is also overdue for an update which I usually do before a re-build.
I believe this issue has been resolved now. I did the aforementioned forum application re-build and it looks like quoting across items is functional once again. While I had the forum down, I also brought the system up-to-date. Enjoy...