I am facing one issue, May you please help ? I do added custom payment gateway its working fine but after payment done customer automatically gets logged out from application. What can be the issue. I also tried disabling Full page Cache but still facing same issue. I am using Magento 2.3.7 version.
It seems, your issue is related to the SameSite issue. Please refer the below links.
Solution: To solve the issue, contact the third-party service provider and request their developers update their integrations to configure cookie parameters.
Temporary workaround: To make your integration work while developers of the third-party service provider resolve the issue, you can set SameSite value to None. This can be done by configuring headers in Nginx or configuring this parameter via HTTP headers.