I haven't experienced any issues with the Fluid Brush tool yet and I've tested it with the tackpad on my MacBook Pro, Wacom Intuos, Cintiq displays, XP-Pen displays and an Xencelabs drawing tablet. Have you tested the Wacom drivers? I assume they are up to date if you haven't experienced issues in other drawing programs. But it just may not be Animate. Perhaps your system resources?
I am having fluid brush issues in Animate 2022 as well. I am trying to follow along with Intro to Adobe Animate https://www.youtube.com/watch?v=-woaDyBXkyU , but when I try using the fluid brush at 11:44 in video, whether I use my wacom tablet or mouse, the part I paint shows up until I stop painting and then disappears from the screen. The brush tool works, however. Have spent over an hour trying to troubleshoot, research, reboot.