Problem with local installation of `p4est`

There is a problem with the elixir under examples/p4est_2d_dgsem/elixir_advection_nonconforming_flag.jl, when I run it with a custom installation of `p4est`. In order to tell P4est.jl to use the custom `p4est` installation (compiled with MPI) together with a custom MPI version I used the setup explained [here](https://trixi-framework.github.io/P4est.jl/stable/#Using-a-custom-version-of-MPI-and/or-p4est) resulting in a LocalPreferences.toml looking like
```
[MPIPreferences]
_format = "1.0"
abi = "OpenMPI"
binary = "system"
libmpi = "libmpi"
mpiexec = "mpiexec"

[P4est]
libp4est = "/home/jlampert/p4est-2.8/build/build-mpi/src/.libs/libp4est.so"
```

However, running
```
$ julia --project=@.
julia> include("examples/p4est_2d_dgsem/elixir_advection_nonconforming_flag.jl")
```
leads to the following error:
```
Abort: Assertion 'p4est_is_balanced (p4est, btype)'
Abort: ./../../src/p4est.c:2424
Abort
[...]
```
Any ideas, especially @sloede, @lchristm, @ranocha?

Start with examples/p4est_2d_dgsem/elixir_advection_nonconforming_flag.jl and reproduce the failure using the shown Julia command and LocalPreferences.toml settings. Investigate the custom p4est and MPI configuration around the p4est_is_balanced assertion; the issue is done when the example runs successfully with the local installation.

Written by the indexing model from the issue text.