migrate-import --update does not seem to work as expected, if map is not joinable, due to highwater field?

I'm confused by the behavior I'm experiencing when running drush migrate-import with the --update option.

Expected behavior
The drush command description for --update says: "In addition to processing unprocessed items from the source, update previously-imported items with new data". My understanding is this should essentially re-import ALL items; entities created on previous imports will be updated by overwriting with their source item's data.

Experienced behavior
I have a migration for which I have already run the import and status shows all items have been imported. When I re-run migrate-import myMigration --update, I get Processed 0 (0 created, 0 updated, 0 failed, 0 ignored).

Upon querying the map table I found that all rows were marked with needs_update = 1. So that part worked, but somehow the query for source items to be processed is returning nothing...

My guess what's going on
I did some digging and in MigrateSourceSQL::performRewind() I noticed the condition on OR needs_update = 1 is added only if the map table is joinable. However, the condition on the highwater field is always added (when the highwater field is defined). So therefore, some of the items marked as needs_update could be excluded from query results because they're "below" the highwater mark. As far as I understand, the corresponding map row's data is added later in MigrateSource::next(), which pulls in the needs_update status, but that doesn't help for the rows that were already axed via the highwater condition in the query.

So, is this the intended default functionality for the --update option? I did find #2312075: Add an --ignore-highwater option to allow full update without having to manually change the db which unearthed the highwater limitation here, but its patch providing the --ignore-highwater option feels (to me) like a workaround to make --update behave as it seems to be documented, i.e. operating on all items. If --update is designed to only include items that are new/updated since last run, that seems to duplicate the basic migrate-import behavior.

I'm categorizing this as a support request in case I'm not understanding the intentions here. Thanks in advance!