Ofi,
I slightly disagree with your conclusion: "The issue is only concerned with SP Page Builder." This is a false statement.
Even the error messages in my Inspect Consol pointed the finger directly to Helix not SPPB, but likely coding in both.
My personal tests revealed when the bugged SPPB was installed my WHOLE websites had E404 Errors even pages NOT created with SPPB.
Bascially, the light in your office did not work (SPPB/Helix Ultimate) as well as the WHOLE building that your office is located (Joomla).
This is why CMS Junkie reported that their own directory's webpages received E404 errors. Bascially, SPPB is causing issues OUTSIDE SPPB created pages.
This is what everyone on the forums are saying: The 2nd tier links in their "directory" pages did not work. These pages are NON-SPPB created pages.
My test also revealed that the SPPB pages DID work, however, the 2nd tier links did not work.
When, Helix was DISABLED the issue STILL existed on the Default Joomla tempate on ALL the pages I tested, SPPB created and NON SPPB created pages. Bascially, testing a non created SPPB webpage on Joomla's default template.
That is why I asked the question as the problem EXISTED on a NON Created SPPB page on a NON created JoomShaper template.
This is an import factor for the develoers to understand; so, it does not happen to us again.