We received an email from Jawomo, the developer of bxActions, telling that his users with T-Mobile Galaxy S8s have been reporting that the recent June security update broke the remap functionality. As a T-Mobile Galaxy S8+ owner who uses the BixBye app and just updated, I realized that I had been seeing a lot more of the Bixby setup screen than usual, but hadn't connected it to the fact that Samsung had yet again blocked remapping apps from doing their jobs. I installed Bixby Button Remapper and Custom Bixby Button just to make sure that all of these remapping apps had been blocked, and sure enough, they don't work anymore. Samsung has implemented a workaround for the devs' workaround.
Currently, only T-Mobile models are affected, with other variants with June security updates still able to use these apps. Jawomo doesn't plan to give up this fight and is already looking for another workaround, so we'll have to stay tuned.