[...] my phone received a notification from Gmail with an automated email from Kraken for a withdrawal, but after 2 seconds this notification disappeared as if it were some kind of bug [...]
Obviously not a bug, the attackers deleted it, they have full access to your gmail account as well as your personal life.
Say goodbye to your money, but what you should do now is:
1. reflash and reformat your both phone and PC because one of them is infected with a form injecting malware targeting cryptocurrencies, your case is very common lately
2. change your Kraken password and make sure it is not used on other sites (only after the 1st step, changing your password right now will not help)
3. enable 2FA in your Kraken account. Would not helped in that particular case because they might control your phone and read sms, but still you should use 2FA. I would recommend buying a separate cheap 15$ phone and sim card for receiving 2FA sms exclusively, because it looks like in your case this can likely happen again.