I have only tried it with low carb bread, the Livlife one. I ate a sandwich using it fresh, then tested, and a couple of days later got some of the frozen bread and made the same sandwich - not a bit of difference.
I think that the liver may be playing with you here. I think my reduction success is due to following a regular process that has depleted my livers ability or desire to maintain a higher level, so your single spot check could be misinforming you. I have been using frozen toast technique for several months now. My 3 monthly average today registers 5.8
In comparison, when I was following Eatwell as directed by the NHS, my daily averages were in the range 15 to 32+ (KETONES or HI shouts my meter) and I was taking 320 mg Gliclazide and 2000 mg Metformin at that time. I am now down to 1000 mg Metformin and 40 mg Gliclazide.
When I was in Hospital last year for some 5 months, I was back on Eatwell fare, and discovered I could eat the toast as an alternative to the Cheerios etc that comprised breakfast. I was able to choose limits to the other carbs in the meals, and got my bgl back down even on hospital fare. I did a follow up, and found that the bread used for the morning toast came from the freezer.