I have just tested this on 6.8.6 and it appears to work on that, but fails on 7.6.0. As such I suspect this is either a bug (The new settings UI, incorrectly ignores this setting), or incorrect documentation (The new settings UI was never meant to support this setting, despite it be documented as an either/or option under the 7.6 docs).
I have raised an issue on the GitHub page https://github.com/elastic/kibana/issues/58356, however is anybody aware of any potential workarounds for this? I have tried extracting the "object" using Infrastructure UI issue, and importing it via the ndjson file, but it appears to be the wrong format. Basically is there an alternative way to set the defaults values, or import the value for this setting.
i.e. Something like:
{"type":"infrastructure-ui-source","id":"infrastructure-ui-source:default","attributes":{"name":"Default", "logAlias":"logs-*"}}
Thanks,
Emmet