I installed the same Windows LTSC (ver. 1809) in another VM and can confirm that SRP works well. So, I have two LTSC VMs - one with working SRP and one with not-working SRP.Tried LTSC ver. 1809.
SRP did not work, both via GPO or H_C. The LTSC is not a complete Windows Enterprise edition.
SRP works well on Windows Home, Pro, Enterprise.
This strongly suggests that the "incompleteness" of LTSC does not have an impact on SRP.
In my case, the issue is probably related to my tests on the Child account (over a year ago) which is known to be incompatible with SRP.