They could be LFP, but I just doubt it because mechanical footprint, BMS optimization for two different chemistries in the truck etc... But anyways 80% of 450 miles is still 360 miles.
Its not that deep. If the advertised 100% charge range was 450 miles then at 80% it will be 360 miles whether the RE is charged to 100% or not. And to add, it's healthier to only charge LFP to 100% once per week instead of every time. I also highly doubt the range extender would've been LFP.
If you're going to share the 80% charge range without the range extender, you should also share the 80% charge range with the range extender. Your post has no credit because of this. You shared 80% charge range of 240 miles, so use the 80% figure with the extender too, which is 360 miles.
Does it give you the option to go over 24A when recommissioning? If not I would try commissioning via the Tesla One app, and then commission on the regular Tesla app afterwards. I had a similar issue once whenever my wall connector got a FW update.