Personalize Import job requires 1000 interactions even when INCREMENTAL?

It appears as if Personalize's CreateDatasetImportJob requires a minimum of 1000 interactions even when running in INCREMENTAL mode ... is this expected? Possibly a bug? I'm getting job rejections when trying to add batches of events; the batch size itself is variable and is controlled by AWS Glue, which periodically will create files with fewer than 1000 events :-|

(I completely understand the 1000 interaction lower-bound when using FULL mode, but here I'm attempting to add to an existing dataset incrementally.)