Solution and conclusions:
After multiple tests, the conclusions are as follows:
1) In many Linux distributions (mine is Alamalinux 9), the default driver is rtl-sdr-v-0.6. With this driver, RTL-SDR-v3 works perfectly and the H line is detected. However, RTL-SDR v4 does not work. It doesn’t throw errors, but it doesn’t detect anything, and the graphs don’t look right.
2) I uninstalled the default 0.6 driver and installed the latest version from https://github.com/rtlsdrblog/rtl-sdr-blog.git. I compiled and installed it, and now both v3 and v4 work perfectly.
The problem, and the reason for so much time spent on this issue, was that with the standard 0.6 driver, v4 didn’t throw errors and seemed to work. Fortunately, the issue is resolved, in case anyone is interested.
Miguel