@brasskey I always fill my main at the station first, with subtank either empties or having filled my main via transfer....then I fill the sub...
Try filling your main first...with sub empty...and see if it matters. I don’t know that it is the key, but it’s my habit now and I’m not seeing problems.
If it turns out this does help...simply get in the habit of routinely transferring while driving when you’ve got room in the main.
Give it a try. I’m curious now.
On the same token, I’ll try reversing my description and see it I’ve hit on something that’s leading to success...or if it just works for me for some other reason.