With Flame 2024.1 and Vulkan, we now cleanup matchbox GLSL shaders on load time to make sure they are compatible with the more modern Vulkan API for backward compatibility.
We did not plan upgrading the supported GLSL version for Matchbox in Flame 2024.1, although this is definitely something we could plan for a future release if there is enough requests from the community, so I highly suggest you open a Feature Request for this.