Googling revealed that SBT sometimes could have a hard time detecting snapshots with date stamps in their filenames ... but that was back in sbt v0.13 days, and besides, my mvn project didn't find that spansphot either!

Another kotlin project, with pure javascript output, is in use in the same sbt project. A newly published snapshot from this project was imported with no trouble into said sbt project, except that also this pure JS project depends on the original MPP, and so SBT was back to not finding the underlying mpp snapshot.

Wait ... the JS project is a gradle project ...

Finally, the 1.4 release announcement had a pointer to https://kotlinlang.org/docs/reference/using-gradle.html, where the kotlin gradle plugin is said to be imported thus:

plugins {
    id 'org.jetbrains.kotlin.multiplatform' version '1.4.10'
}

Oh ... I used to have just

id 'kotlin-multiplatform'

...

Changed the id and re-published. Now imports into maven and sbt projects like a charm. But I did have a hard time tracking that down!