RE: XPS 15 9550 - TPM has "disappeared"
This just happened to my XPS 15 9550 as well. I had BitLocker enabled using the TPM since June 2016. I used the laptop over the weekend just gone and when I turned it on this morning (Monday 1st August) the TPM was gone and I had to enter the BitLocker recovery key.
I tried updating to BIOS 1.2.10 and nothing changed. I also tried reverting to BIOS 1.1.15 and it still did not work.
I honestly suspect Dell has forcibly disabled the TPM chip in this device due to accidentally enabling it when they did not intend to do so in the first place as it is not a "business" device. This seriously pisses me off because I use it for work and part of the reason I bought it was because I knew TPM worked (after checking other owners for their experiences).