RE: XPS 15 9550 - TPM has "disappeared"
I had the same symptoms when upgrading from 1.1.15 to 1.2.0 and the only way I could resolve it was to:
1) Enter the recovery and back up everything you need (i.e. prepare for a fresh install).
2) Roll back to the old BIOS (for me it was 1.1.15) - which should make your TPM re-appear.
3) Re-install a fresh copy of windows.
4) Upgrade your bios to the latest version.
5) Check that the TPM can still be seen.
6) Encrypt again.
It worked for me and might work for you. I just hope I don't have to do it for every BIOS update!