I’ve continued to troubleshoot… First, an answer to your last question: If all Rules are disabled, does it work – No.
Continuing to troubleshoot, I’ve disabled one DFFS feature at a time, and have found a repeatable symptom – When the Cascading fields are placed on a row with other fields using SBS, the results become very unpredictable when Hide the dropdowns when they have no available options is turned on.
The symptom is that the top-tier cascade field will not display (though a screen refresh may bring it back), the second-tier will not display.
I kept SBS on, but moved the two tiers onto separate rows without any other fields, and the Cascade functions all worked perfectly.
When I returned to my preferred layout, where top-tier, second-tier, and another non-Cascade field align on a single row, the symptom returned.
In the attached screencaps, the fields LOB and SubLOB are Cascading.
As always, your support and efforts are greatly appreciated!
Dynamic Forms for SharePoint v126.96.36.199 – March 13, 2019|CSS version: 4.46 / 4.46|spjs-utility version: 1.332|SPJS Cascading dropdowns v3.7.25