Fullscreen and Windowed use my primary monitor, Borderless Fullscreen uses secondary monitor and constantly swaps back to secondary when loading servers or games. The list of monitors only ever displays one monitor and the indicator doesn’t change regardless of what screen it is on.
Other UE4 games have this bug (though I’ve not come across them) so it’s not a TU-specific thing. Aven Colony and Battlefleet Gothic Colony 2 users report the same bug. Someone’s suggested “-WinX=0 -WinY=0”, which if works might be worth setting as default? [Edit: those launch parameters make no difference to screen or position]
A UE4 bug report marked fixed with 4.20 has a similar issue where if the right monitor is primary it’ll always open on left (not the same as my issue as left is my primary). https://issues.unrealengine.com/issue/UE-54135
A developer who’s had the default monitor issue with 4.18 and wrote a fix for their case https://www.froyok.fr/blog/2018-01-ue4-specify-default-monitor-at-launch