Why worry -- other than the fact that it's flammable. All that's needed to start a fire is having the thermistor fail or come loose. I'd use a flameproof material if you're going to insulate.
The flash point (ignition temperature) of cork is, apparently, 300 - 320°C1, which is not, as far as I can tell, a temperature that the heatbed reaches, so, in theory, cork should be safe to use as an insulator. In fact temperatures of around 300C are used in the manufacturing process of some cork products2:
For insulation applications, agglomerates of granules of cork, known as black agglomerates, are employed. They are manufactured in a closed autoclave at high temperature (approximately 300uC) and pressure (around 40 kPa) without the use of adhesive
In addtion, according to Why should we use cork?
Does cork burn?
Cork is a slow combustion material. That is to say, yes it burns but very slowly and it doesn't produce flame so it doesn't spread. Also, when burning, the smoke that it releases is not toxic.
However, I am not sure if all cork is equal, or whether the thickness of the cork can affect the safety. To give a definite figure, I was thinking of using 2 mm - 5 mm thick cork sheeting.
Has anyone experienced, or know of, any burning (or scorching) of cork, when used as a backing insulator to a heatbed, in particular, an aluminium PCB MK3 heatbed?
The real fire risks with 3d printers are electrical in nature. Lets say your wires come undone and happen to come next to the other lead. If it archs and happened to be in just the right position. Pretty unlikely. Once I did a bad solder job and when I was working on my printer. The wires literally burst into flame in my lap. No damage done (other than needing a new board). That said we often see people who have their boards catch fire in the flashforge owner groups. Weak solder joints, and over all bad quality.
Do you really have something to worry about with Cork? No. A series of unfortunate events would have to happen, and more likely your board will cause a fire. That said I would look into adding a layer of aluminum and reflect some of that heat back up.
If you are that worried I would just remove the sheet. Unless you are having issues calibrating you heated build plate / PIDs there is no reason to use insulation. I only use it on printers that have a hard time hitting ABS temps of 100c. Even then I only use tin foil and try to trap the air underneath.
Just a side note. If you are having heated build plate issues maybe what you really need is a heat chamber.