Updating object added with cache.modify doesn't trigger watchQuery update (has failing unit test)

[Narretz]

Intended outcome:

I have a watchQuery with fetchPolicy network-only and nextFetchPolicy cache-first that returns an object with initially empty things.
Later, I add things to the object via cache.modify.
The watchQuery is triggered, it now contains the things.

Later, I call readQuery which updates one of the things with a new value.

I expect the watchQuery to be triggered again, with the new value for the thing.

See this unit test: #9439

the documentation says:

Like writeQuery and writeFragment, modify triggers a refresh of all active queries that depend on modified fields (unless you override this behavior by passing broadcast: false).

https://www.apollographql.com/docs/react/caching/cache-interaction/#using-cachemodify

Actual outcome:
After readQuery, the watchQuery is not triggered.

It is however triggered if I use cache.updateQuery to add the things.

That leads me to believe that this is a bug.

How to reproduce the issue:
I've added a failing unit test here:

#9439

Versions
Unit test ran against 3.5.9