From what I can tell in an initial investigation, based on the list of jars that are downloaded during the baseline task, it looks like the baseline task is checking against future releases (rather than the previous release) of an artifact to decide whether breaking changes have been added.
When you compare against the future, this means that new methods and classes added to a future release will look exactly like deleting a method or class, and baseline decides that the packageinfo files all need a major version increase.
The baseline task will automatically update all of the relevant packageinfo files because the default baseline jar report level is "persist". This causes the snapshot artifacts for the root level projects (portal-impl, portal-kernel) to be installed with new packageinfo versions that have a major version increment, which in turn causes com.liferay.portal.bootstrap to claim to export package those updated package versions.
As a result, almost all modules relying on any of those packages will fail to start (they're all being exported at 1 major version higher than they were at release time, which means none of the auto-generated Import-Package statements would include that new version in the version range), causing your bundle to fail to start.