I am not sure — it has crossed our minds before but we never made even levels a requirement for the format because maybe there are use cases when an uneven taxonomy would be desired? This is why we ultimately decided to catch this with warnings in plugins that require it, rather than enforcing this in the format itself (so that it would be caught during import or when an artifact is generated). It’s certainly not too much work, it has always just been a philosophical question of whether taxonomies must be even-ranked… my gut tells me no.
On the other hand, we could make this a property so that some actions explicitly require an evenly ranked taxonomy, others do not. Catching this with a warning is much less work in the end I think…
Glad we could solve your issue! Happy 
ing