Dataflow can not be saved - API + Sharepoint (I assume formula.firewall issue, but not clear)

[NovaBI]

Did anybody successfully establish a dataflow that is using an API and another source (like an internal sharepoint library e.g.) - we have the issue that this seems to cause an formula.firewall error (API is public security level, the other source must be organizational at least). We can set up the flow and it is working as expected, when we try to save it, we receive an "internal error" message, nothing else.

I am pretty sure that this is caused by the logic of how the partitions are used in PBI, does somebody has a proper solution for it (it works with different security levels in PBI Desktop as well, but turning on the option "ignore security levels" is not a viable solution.

Did somebody tried to flatten part of a query e.g. successfully? Using a function to bypass the formula.firewall but keeping the security layer is also not working.