It seems that InsertOrUpdate generates a command with two statements:

[info] 2024-06-15T18:34:30.527 [-2] DEBUG slick.jdbc.JdbcBackend.statement Preparing statement: update "test" set "value"=? where "id"=?; insert into "test" ("id","value") select ?,? where not exists (select 1 from "test" where "id"=?)

This will result in lost updates when running in Read Committed (the default in many databases) because another transaction can insert the row between the two statements. It will be ignored by the update that happens before and ignored by the insert that happens later. Because Read Committed on Multi-Version Concurrency Control sets a different read time for each statement.