Yes eating flax seeds a few days before a drug test could cause you to fail the drug test.
I love flax seeds and ate some before I had to go in for a drug test for a job and I failed the drug test and I told the job that I had eaten some flax seeds before then and they said it was likely due to the flax seeds.
They said they would reconsider hiring me and to avoid eating any flax seeds at least a few days before the drug test were to happen.
When I went in for the second drug test and avoided the flax seeds I passed the drug test without any issues.
So as long as you don't actually do drugs and avoid flax seeds and many other seeds before the drug test then you should have no problem passing the drug test.