This indeed looks like the same race condition discussed in ACME fails with JWS verification error · Issue #101445 · NixOS/nixpkgs · GitHub
The ongoing discussion in nixos: mutually exclusive services; application to acme by symphorien · Pull Request #102387 · NixOS/nixpkgs · GitHub will result in a solution but in the mean time, a reliable workaround is to add aliases to the “email” parameter for each cert. For example, in your case you could use kontakt+dovecot@example.com. This will create an accounts directory for each cert. Bare in mind there is a 5 accounts per day rate limit.