I have the exact same problem.
Luckily I have two FP4 phones at my disposal right now (mine and my dad’s one). Interestingly my FP4 with /e/OS is fine (I’ve also installed it by myself). But the phone of my dad’s has this data-connection bug. He has the original stock android. This should be enough proof that this happens to stock Android as well.
There is at least another guy that has the same problem:
One more possibly useless distinguishing thing: my FP4 has only 6GB RAM, the problem one of my dad’s is the 8GB version.