I have three devices: A Windows 11 PC with Chrome/Firefox/Edge, an iPad (iOS) with Chrome/Safari, and an iPhone (iOS) with Chrome Safari. With any of these devices, with any browser, you used to be able to post to a Facebook group using the Share dialog. On or about 3/7/24, it stopped working for non-mobile devices. This can be easily demonstrated on each device, using this URL: https://www.facebook.com/sharer/sharer.php?u=https://google.com
Try it on a PC - you'll not have the dropdown that will permit you to select a group, but you will on your cell phone.
Here are two examples of share attempts from my website, one using a PC, the other my iPhone.
