I have this problem, too, but only after the latest Orion update (Version 0.99.126.4.1.3-rc (WebKit 618.1.7)). It had been no problem at all before.
Basically not a single web site that offers to remain logged in works anymore in this regard; after a short time (hours, max. days) I find myself logged out (usually I remain logged in for months or years). Examples include Google, Wikipedia and orionfeedback.org 😉.
Compatibility mode does not help, however, a clean profile does seem to help. I will watch this for some time and report back.