I updated my test to output the uuid’s:
insert: a0569da7-4201-47a8-bab0-4aca263b735a => ok
insert: cc2d9293-5898-4118-8352-c192090bf6ef => fail, got new id: 3f2d3f3f-583f-4118-3f52-3f3f090b3f00
another run:
insert: 2361f715-48f9-4574-9cb2-379883a8adc4 => got new id: 23613f15-483f-4574-3f3f-373f3f3f3f00
interesting: the new id’s are sharing some parts with the original ones…
/e: now found this: binary id odd bug · Issue #2602 · elixir-ecto/ecto · GitHub
i’ll try that later, hope it fixes the problem 
/e²: Fixed it by adding utf8mb4 to the repo config (config/dev.exs):
# Configure your database
config :testproject, Testproject.Repo,
username: "xyz",
password: "xyz",
database: "xyz",
hostname: "xyz",
charset: "utf8mb4",
collate: "utf8mb4_unicode_ci"