Importing pdf - keeping layers intact

Corel Technical Suite 2018, Windows 10......We work on pdfs that originated as dwgs and import them into Corel to work up graphically. 

Imported pdfs might have, say, 12 layers, but once imported into CorelDraw (or Designer) each individual layer explodes into between 2 and 40 separate layers. And they don't come into Object Manager in order either, layers with the same name are all mixed up with others.

Anyone got any clue why this is or how to make sure the layers in the pdf come across to Corel intact?

Thanks