Why should there be a ratio? Why not just all segwit or all taproot?
BTCPay clients only support a single script type, and Trezor clients previously only supported Taproot. In order to provide sufficient anonymity for both, Wasabi clients choose both script types randomly.
Is it a bug you and I are just aware of, or is there some Wasabi developer aware as well?
The devs are aware. I suppose I could open an issue for it, but it seems like a pretty harmless bug.
Have you investigated the source code yourself and not found what goes wrong?
I'm not a programmer, so I'm unable to audit the code myself.