Inconsistent behaviour when specifying possibleTypes

[ramiel]

Intended outcome:
Apollo should decide to crash (or not) when trying to modify the store and a property is missing, in a consistent way. Either it always crashes, either it doesn't.

Actual outcome:
Apollo v3 crashes if I try to modify the store and while doing this I omit some data. The problem is that it crashes only if possibleTypes is defined. The behavior (crashing or not) shoulnd't be related to the presence of possibleTypes or its absence. It's hard to see a correlation between these two aspects and adding (or removing) the option lately just break the existing code because, up to that moment, the behavior was the opposite.

How to reproduce the issue:
An example it provided in this issue: #6915

Versions
System:
OS: Linux
Binaries:
Node: 14.17.0 - /usr/bin/node
Yarn: 1.22.10 - ~/.npm-global/bin/yarn
npm: 6.14.13 - /usr/bin/npm
Browsers:
Chrome: 91.0.4472.77
Firefox: 88.0.1
npmPackages:
@apollo/client: ^3.3.19 => 3.3.19
apollo-upload-client: ^14.1.3 => 14.1.3