Ultimatum not spawning on white map despite over 100% Atlas chance

I have over 100% chance to encounter Ultimatum from Atlas passives.

I opened a white map:

No scarabs

No Kirac map device mods

No other league mechanics forced

The map completed without an Ultimatum encounter.

This is not the first time this has happened.

On a white map there should be no encounter cap conflicts, so with over 100% chance Ultimatum should spawn every time.
If encounters can still be overridden or capped internally, then the Atlas wording is misleading.

Just to make sure this wasnt a Kirac mission was it? Kirac missions ignore atlas passives.

It wasnt a Kirac mission