SUMMARY
When automatic adaptive sync and tearing are both enabled, legacy modesetting is used, and the monitor is set to 240hz, pushing framerates above the monitor's adaptive sync range causes stuttering instead of a graceful fallback to tearing.

STEPS TO REPRODUCE
1. With atomic modesetting disabled, configure monitor to output in 240hz (actually a fractional value, I can check when I get home, I think it was 239.76), with automatic adaptive sync and tearing enabled.
2. Run a Vulkan game in fullscreen that pushes a framerate that exceeds the monitor's adaptive sync range. I used the enhanced Steam version of Quake running at ~1600fps, but much lower framerates trigger the issue as long as they exceed 240fps.
3. Move the mouse.

OBSERVED RESULT
Stuttery, visibly low-framerate gameplay and cursor movement and dropped frames even when max frametime is very low.

EXPECTED RESULT
Smooth gameplay with tearing.

SOFTWARE/OS VERSIONS
Linux: 6.16.8-arch3-1
KDE Plasma Version: 6.4.5
KDE Frameworks Version: 6.18.0
Qt Version: 6.9.2
Mesa: 25.2.3-arch1.2

ADDITIONAL INFORMATION
CPU: Ryzen 7 5800XT
GPU: Radeon RX 9060XT 16GB
Output connector: DisplayPort
Environment Variables: KWIN_DRM_NO_AMS=1

The game in question uses Xwayland.

Removing any of the offending variables independently fixes the issue. Lowering the refresh rate to 144hz or disabling adaptive sync both fix the issue and tears, and disallowing tearing removes the stutter but is suboptimal for use case.