I don’t think the tight lid was the problem. Nor your water, unless you have very high fluoride content or some other contaminant. The seed mat, or the other hand, is probably the culprit. I don’t know how you set it up but I would guess the temperature might have been too high. Also you would not have wanted your jar to be sitting directly on the mat, as the direct conduction of heat to the jar would have caused the jar to heat more than surrounding air. The grains should not exceed 90 degrees Fahrenheit. The grains are really quite resilient and cool temps will slow them but not kill them. To high a temperature will definitely kill them.
Let us know how you make out.
Rick