Basically this thread here covers an example of the issue. It seems to only pop up when running multiple iterations of a simulation, i.e. using BenchmarkTools all allocations are zero etc.
When I went away from using StaticArrays and only used 1D vectors, this issue with GC for updatexᵢⱼ! (in the link) would be gone - but not when using StaticArrays. So I am suspecting some issue with StaticArrays here - perhaps from my use.
Kind regards