I think you are throwing a great deal of water on this, but, it sounds like it could be independently testable, and verifiable. Now, we might need to know more about what make model of phone was in use, as well as apps, provider, etc, to replicate this. However, this could be turned into a test campaign with a test plan, treating this as any SUT. To determine if there is any substance to it, of course.
Given some of the more recent exposures of "Phone always-on behavior", constant telemetry [1], apps violating policy, FB&Google collusion around bid rigging, bypassing privacy laws, and, data being sent to facebook whether or not it was intended to go there, it sounds like it is within the realm of technical possibility, or, that it is occuring in some situations that are not readily apparent, or a bug.
Maybe it isn't occuring too, that is a possibility also.
Given how much of the internet is trying to share data back with FB, Google, Apple, and many others just by looking at Umatrix/Ublock, and a few other privacy apps, there is no doubt that much of the mobile internet is really feeding data back to them all the time. Good luck controlling that on mobile though! And, I am not talking about back-end transactions either..
[1] https://www.tomsguide.com/news/android-ios-data-collection