The default OpenBLAS packaged in Archlinux can produce wrong results for float64.
This is due to the fact that in Archlinux, the blas package (from netlib reference implementation) has been separated into blas for Fortran + cblas for C interface. And Openblas only replaces blas but uses the netlib cblas which seems to be incompatible.
Looking into the standard library, it uses cblas and so will be affected by this bug.
Upstream: FS#63054 : Openblas built from community gives wrong matrix multiplication results (DGEMM)
Workaround: use one of the openblas-lapack package from the AUR that provides Fortran blas, C cblas and Lapack interfaces.
Official build of Julia (or if you build Julia from source using BinaryBuilder-powered binaries) ships its own version of OpenBLAS, though. On ArchLinux I use the julia-bin package in AUR for the stable version of Julia.
Could you please check if you can reproduce when using the library in https://github.com/JuliaBinaryWrappers/OpenBLAS_jll.jl/releases/tag/OpenBLAS-v0.3.7%2B0? Just find the tarball for your system (operating system + fortran library) and (temporarily) replace your system libopenblas with the one in tarball (need to rename it, as it has a trailing underscore).
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Thanks! I wanted to highlight that in fact it works to use the supplied OpenBLAS for future searches…
Note that installing the OpenBLAS_jll package did nothing for me - linking the lib however did.