If microwaves heat from the inside out, why is my reheated lasagne always cold in the middle?

I don't think microwaves heat from the inside out. Things heat up because water, fat and sugar molecules absorb energy given off from mircrowave radiation.

https://en.wikipedia.org/wiki/Microwave_ovens

Perhaps a closer look at the 'Uneven Heating' section of that link might be useful.

Microwaves definitely do not heat from the inside out. The waves penetrate the food and heat at the middle, but they heat more at the outside.

Normal heating works by heating the outside only and heat travelling through by conduction to the middle. So microwaves are better at heating the middle, but they still heat the outside more.

Some microwave ovens have "cold spots" in them which don't get heated, so manufacturers have devised turntables which turn the food, and "stirrers" which stir the microwaves around causing them to bounce around at random in an attempt to eliminate the cold spots.

microwaves dont heat from the inside out!! i kno evry1z sed it alredy bt im doin GCSE physicz n am jus proud 2 say thatz 1 thing i do actully kno about it!! :D

One often hears the statement, "microwaves cook foods from inside." This is because foods heated in a microwave tend to heat most rapidly in the center, and cooks much faster than what we intuitively believe from conduction of heat from outside. The statement, however, is not entirely true; microwave radiation simply has the ability to penetrate several inches into the food. Radiation with longer wavelengths tend to penetrate deep into matter, evident by the ease by which radio waves go through concrete walls. Since microwaves penetrate into food several inches down, it can heat foods quicker than if it were being heated through conduction by boiling water on the outer surface. If the food were larger than several inches, however, the middle of the food still needs to be heated by conduction.

https://physics.stackexchange.com/questions/613321/in-a-microwave-oven-why-do-smaller-portions-heat-up-faster
I stand by my question, my lasagne wasn't too thick for the microwaves to reach the centre.

In many cases, the instructions for microwaving foods say 'Leave to stand for ...' a certain time. This allows the heating to even out through the product. Also, the cold spot can be a problem if you don't have one of those old-fashioned microwave ovens with a revolving turntable. Try repositioning the stuff part way through cooking if yours doesn't revolve. One reason for the idea that things cook from the middle is the doughnut effect. Ever microwaved a slightly old doughnut to refresh it? The jammy bit is somewhat hotter than the rest because certain substances heat up in microwaves quicker than others.
(By the way, I hope a certain Princess doesn't spell like that in her school work...)