Lamia

when the game is force reset, it is essentially a game crash. it is important to note EXACTLY when the crash happens. it could be anything from improper dialogue coding to the map not being inserted correctly

modify the editor to use a different location in the ROM where you have enough empty space

there are already two chapters that have this function in FE7 where the chapter ends when you go to a tile (more like Arrive though)
if you want to have proper escape, it should be easy enough to have it where, if character reaches tile, display dialogue text, then use a command to move that character and disappear, and you could make it based on Arrive instead of Escape if you wanted too
you'd most likely have to set an event for each possible escaped character for the chapter if you want some special dialogues, but that's easy, just time-consuming
if you want to only have a chapter victory if all your units escape, you might be able to set flags to indicate such so that when you control 0 units and flags are set, then the chapter is a victory
uhh, keep in mind this is all just conjecture, I've never worked with GBA events

bit of a headscratcher that FE7 and probably the other GBA games only have one character byte, I suppose they were more efficient (or more lazy)
in FE4/5 there are two bytes and it allows for that higher limit (65535) despite having much less ROM space, although there's data for only about 600 characters at most
I would suggest leaning on the end of more efficiency if you find yourself with a shortage of character space

I have run across random bugs like this in my own project long after the time that I originally caused it
1. make a savestate
2. check earlier backups of your ROM until you find the newest one that does not have the error
3. run a file comparison hex editor (like hex comparison) and locate ALL differences between the newest backup that does not have this problem, and the oldest backup that does (or just your current version)
this is very time-consuming but it's worth it in the long run if you've put a lot of work into your project
4. experiment with fixing the crap that caused it
while you could potentially just remove the poison status completely from your project (although this error may also come up with other turn-start effects), mine was more serious in that the game would hang after a stupidly specific set of circumstances (player unit attacks enemy unit at 2+ spaces away without killing that unit, gains EXP, and also does not level up from that battle) but it was still important to fix it on the occasions it would happen

what's this have to do with FE4?
also, consult my signature for important FE4 stuff, and don't hesitate to ask some questions after hopefully playing around a bit

loading a ROM by itself does not cause that error unless you have an ini file created from linking to modules
if you moved anything around you need to modify or delete the ini