Hello again,

I have been debugging this issue with the manufacturer for a couple of days, and found something very interesting.
Initially, I thought it was a System Firmware (BIOS) problem, that it was misbehaving and reporting an invalid log.
But it seems to be a Kernel related problem.

If the system is booted with a customized Linux Kernel 4.19, the TPM Event Log present in sysfs is invalid: tools fails to parse it, and there are no SHA256 measurements.
If the system is booted with a vanilla Linux Kernel 5.8.15 (Fedora Workstation 33 Live), the TPM Event Log is valid, tools can parse it, there are both SHA1 and SHA256 measurements, and the reconstruction matches perfectly with the state of the TPM PCRs

This is more evident by comparing the binary_bios_measurements files taken from both executions. I can see the good one starting with "Spec ID Ev..", and the bad one directly with "Secure Boot" related info

The git history of the kernel source related to TPM Event Log is available here https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/drivers/char/tpm/eventlog. 
I will look there to see if something shed some light to this issue.

Any hints are appreciated,

Thank you for your help so far!