Your accounts are always checking in from the same IP. That's the simplest method of detection. Add some behavioural traits and it's scarily accurate.

If you are using both accounts from the same IP address and/or phone it is trivial for Meta to figure out it is one person using two separate accounts.

Same device, same IP, same kind of behavioural traits. No wonder, however one would think that Instagram is intelligent enough to recommend content based on subscriptions instead of just behaviours.

This is what the fingerprinting your phone is for. They save interests and info about you attached to the fingerprint of your device and share that with any other service that asks to retrieve it, including another Instagram account on the same device.

Edit: just fyi, I do not mean actual fingerprint unlocking, but just a unique identifier they create depending on your setup.

On Android, apps can read information from each other as long as both apps consent. If these apps are using the same libraries to connect to Instagram then that library may be enabling sharing data between the two apps.

I assume the situation is similar on iOS.

Also you've markes yourself as a bot account. Am I talking to a bot?