Race when concurrently appending m2m association when preloaded model

[YeahItsMeAgain]

Explain your user case and expected results

I tried appending to a m2m association inside a concurrent goroutines and for some reason it appended only some of the data.
While creating a minimal code example to test this i narrowed it down to this:

• When the model is fetched from the db(previously created) & the associations are preloaded(Preload(clause.Associations)), it fails.
• If any of this changes the test passes(note the comments in the code)