How reliable is the reference model of cosim?

Nobody has claimed this yet.

I installed the co-simulation system, followed this [tutorial](https://ibex-core.readthedocs.io/en/latest/03_reference/cosim.html#setup-and-usage) to install cosim and tested it on ibex. One of the test cases shows unexpected behavior:
```
FAILURE: Co-simulation mismatch at time 1302
Register write data mismatch to x5 DUT: 2 expected: 1
[1302] %Error: ibex_simple_system_cosim_checker.sv:68: Assertion failed in TOP.ibex_simple_system.u_ibex_simple_system_cosim_checker_bind: Co-simulation mismatch seen
Received stop request, shutting down simulation
```
I would like to ask, how reliable is the reference model of cosim? I am unsure whether this issue is necessarily a problem with ibex because the reference model version is [ibex-cosim-v0.5](https://github.com/lowRISC/riscv-isa-sim/releases/tag/ibex-cosim-v0.5).

Begin with the cosim setup and usage tutorial and the failure site at ibex_simple_system_cosim_checker.sv:68. Reproduce the mismatch at time 1302 using reference model ibex-cosim-v0.5, then establish whether the differing x5 write comes from the DUT or model and record the conclusion.

Written by the indexing model from the issue text.