[jameshowe (OP)]

When I freeze my PET my plastic, (PET), bottle Cola, then thaw it out to open & drink it, there is no fizz or bubbles in it. The drink is completely flat.

[chiralSPO]

The solubility of carbon dioxide in liquid water is fairly high at 0 °C. (https://en.wikipedia.org/wiki/Carbon_dioxide_(data_page)#Solubility_in_water_at_various_temperatures)

But when water freezes, there is no space for the CO₂ in the crystal lattice, so it all gets forced out into the gas phase. If none of the gas is allowed to escape, the pressure will remain very high until the ice melts and becomes liquid water again, at which point the gas will begin to dissolve. However, the rate of dissolution in a non-agitated mixture might be quite slow, so the excess gas is released when you open the top

The more likely scenario is that the rapid buildup of pressure caused by the release of CO₂ when the water freezes causes the seal to fail (it doesn't have to be a catastrophic failure, and it may even return to being air-tight after some of the pressure has been released.

[Bored chemist]

The screw caps on pop bottles are designed to release excess pressure safely.