I’m using TPM library that is part of the operating system (libtss2), so I’m not dealing with the connections directly, I just set up the destination server (TPM simulator) with the environment variable TSS2_TCTI. Handling of the connections happens inside the library. But I’m pretty sure they’re closed correctly, because with the artificial delay added, I have now reached ~10,000,000 operations in a row! Also, I tried monitoring the connections with ss, but, most of the time, when I run ss there are no active connections at all. Very rarely, when I happen to run ss just at the right moment, I briefly see one “established” connection - which is gone as soon as I re-try. So these connections obviously are very short-lived.
I use Debian stable “Bookworm”. Also gave the latest Debian Testing a try, but makes no difference. At this point, it seems that I have to live with the artificial delay, until Docker gets improved… 