I've seen this referenced in other threads such as Ongoing freesync issues 18.3.4 , but there is indeed something broken or different about FreeSync in key instances starting with this driver.
Two reliable ways to recreate the issue: Retroarch with any PAL/50hz game with the only sync method being "sync to game's exact refresh rate", VICE (C64 emulator) which is primarily PAL. It will exhibit judder and even occasional tearing in these instances. A 60hz game seems to be fine in Retroarch. My display is a NU8000 55" Samsung TV, for reference, updated to the latest firmware. These issues are completely cleared up 17.11.3 through 18.3.3.
Of note, I notice the way any hardware accelerated game initializes in these older drivers is different. There will be a bit of a pause or black screen for ~3 seconds before rendering starts, like it's rendering a different way than in newer drivers where it's a lot quicker to start (but occasionally with broken FreeSync as stated). I also notice that your own FreeSync windmill demo has completely broken FreeSync AND Vsync after 17.11.3 as stated in Freesync broken since 17.11.4 yet 17.11.3-18.3.3 will properly FreeSync in every other instance I could throw at it, including Nvidia's Pendulum demo.
I would continue to use 18.3.3 but it has bugs in some newer games so it's not magic bullet. Any insight on why FreeSync functionality has changed in the last year or so? I realize this is hard and fiddly thing to test for, but this is expensive tech, something you market, and a huge incentive for me to stick with your products in the future.