Haven’t been able to return to the lentil experiment as quickly as I thought because I’ve been spending my calorie budget fairly rampantly on things not lentil. Mostly of the chocolate variety. Finally managed to work it in tonight, though.
As I suspected, it seems to be adding the seasoning mix to the cooking lentils and rice that causes the problem — I tried the same amounts of lentil, rice, and water with egg but without any additional ingredients. Result: perfect or near-perfect cooking. There were some crispy bits around the bottom, but that’s typical with this rice cooker.
Ate the resulting mixture with some ketchup-with-balsalmic-vinegar that I’ve had sitting around in the refrigerator for a while. Not bad, but I think the teriyaki sauce works better.