[Quoth]

Sometimes Amazon only gives a previous version when you download again, even when the publisher has only updated. It seems the publisher has to also contact Amazon to enable existing purchasers to get newer version. Smashwords is automatic.
A later date dictionary might not be regarded as an update of the same book, unlike a novel. I've had 3 versions of Magician, no auto update, but going to my content to download via USB, the updates were flagged there.
So worthwhile checking your Amazon Account Content and also perhaps contacting publisher.
However not likely to be much difference to a 2019 vs 2013 dictionary even paper versions. They are also not likely to sell 2019 versions to 2013 - 2018 purchasers on Kindle.

Amazon doesn't seem to decide these things, but Publishers.
There is nowhere on the KDP upload to indicate what kind of update and updated content for an existing ASIN title is. CS has to be contacted by publisher. Also seems true to have series titles actually linked on the web page.