Blizzard have clarified that there will not be a stat squish in Dragonflight, as they are only reworking how Chromie Time scales, specifically in regards to item levels as it will now be 10-60. They also clarified that the current stats on heirlooms on the Beta are due to a bug which will be fixed soon. 

Stat Squish (Source)

We are not doing a squish. The overall goal is for player-characters who are at level 60 today to log in at level 60 in Dragonflight with the same item level and approximately the same stats as they had before.

What we are doing: rescaling Chromie Time. In Dragonflight, Chromie Time now stretches from 10-60, so that players can choose which expansion they’d like to level in, and they can stick with that all the way to Dragonflight if they choose to.

So that’s going to require that we change some iLvls within that range to make it a smooth experience, but the endpoint isn’t changing.

There are definitely some bugs with this on the Dragonflight Beta and the PTR, such as your heirlooms, and we’re going to fix them up.