This sounds awfully familiar to the issue I see.
On my dual-screen setup (primary 2560x1440 to the left, secondary 1920x1080 to the right) I keep mIRC maximized on the secondary; but after every mIRC boot (which coincides with a shutdown/reboot) it opens on the primary screen (still maximized though).

I tried Window > Position > Save, but that didn't have the desired result. mIRC.ini has the following entries (not sure which ones are relevant):
Code:
[windows]
main=2558,1160,163,840,2,1,0

[mode]
normal=-4,-4,1160,840,1,2,2,1,0,1,0,1,1,0,0
state=normal
compact=710,310,260,400,0,1,0,2,1,0,1,1,1,1,1

[fullscreen]
normal=-4,-4,1160,840,1,2,2,1,0,1,1,1
full=-4,-4,1160,840,1,2,1,1,0,0,0,0
state=normal


I used WinAero Tweaker to change the border width on Windows 10 to be less imposing (border width is 1 and border padding is 2), which also requires the Aero Lite theme; so the position of 2558 sounds like it matches 2560 with 2px padding.

Also, for completeness sake:
Code:
//echo -a $os $version $beta $md5($mircexe,2) $file($mircexe).sig $alias(0) $script(0) $dll(0) $com(0)
; 10 7.49 97c5ea9ce0093eef4776a5ec6ab12d69 ok 9 32 1 0