No notifications on specific applications iPhone and Apple Watch

Hi, this is my first question here and I am truly desperate. I have a major notification problem on both my iPhone 12 Pro and my Apple Watch series 4, that are linked to the same Apple ID. I also have a MacBook Pro and an iPad Pro that don't seem to have the same problem.

The problem in question began a few months ago when I noticed that I didn't receive any notification anymore on my phone and Apple Watch, from very specific applications that are not "native" from apple such as Instagram, WhatsApp, Spark, Gmail, Snapchat... The problem seem to occur randomly, a few hours after I turned on my phone, and it affects all of these application on the same time. I've been waiting for every single iOS update to fix this problem, but the problem was never fixed.

I also noticed really odd things that i'd like to share, directly linked to this issue:

• On instagram, I only receive some specific notifications from accounts when they're launching a live, even if I havent turned alerts on for these specific accounts
• Whenever I receive a text on iMessage or on messenger (which I still receive notifications from), I can't "swipe down" the alert to answer the text or to see the whole text in question

I have spent multiple hours on phone with apple engineers who were ironically asking me if my notifications were turned on in my settings (as if it wasn't the first thing I had checked when I noticed the problem...), if my devices were updated on the latest iOS version, if I hadn't activated "do not disturb"... I also deleted and reinstalled every affected app, and reseted the entire phone to a factory state and then re-downloaded everything. None of the other tricks out there appear to work (bluetooth, airplane mode, watch notifications, do not disturb, etc.). I even created a new Apple ID to check if the problem would come from my account, but the same thing happened. I can't count the numerous times I've reseted all of my apple products (including the iPad and Mac) but the issue is still happening. This is so frustrating and irritating.

I did some researches and in my opinion, the problem would possibly come from several sources:

• Either the apple one subscription that I have with my family since my mother seems to have the same issue as mine on her iPhone 7, but the apple engineers don't share this opinion
• Or from one specific app that could trigger the problem
• Or from an internal hardware problem that me and my mom coincidentally share on both our phones
• Or from a glitch created by the combination of my devices linked together

I don't know what else to do. I am supposed to sell my phone in the next several months and I am really worried about this problem, it really affects my professional and personal life. I know that im not the only person with this problem which is why I really look forward to apple solving this issue.