The phenomenon is caused by wrong item data being written to the address of the transporter.
For example, if data such as 0xff is written, there are items that do "not exist item" in the transporter.
Have you adapted breaksave patch and recreated saved data when it exceeds status 31?
Since it becomes incompatible with the existing saved data, it is necessary to erase it and recreate it.
Do you adapt SkillSystems?
There was a time when SkillSystems was in conflict with the extended transporter address.
In order to avoid this, I move the address considerably behind the address of the extended transporter.
In the version after February 18, 2018/02, if break save patch is applied, the transporter is placed at the following address.
Name:Status 31 limit break_save
Please check if other patches you installed will not break this area.
If there are no items in the transporter, this area should be filled with 0x00 0x00.
When items are put in the transporter, 2 bytes of information of byte ItemID, Count is written.
Also can you make the game have mulitple main characters (like FE7) so if one of them dies it's game over?
Since FE 8 has a switch between Eirika and Ephraim parties, I think that it can be realized if two hero.
Of course, It can not do it easily.
For example, with a midori(green) patch, an event that refers to one party occurs halfway.
If you want to have three or more heroes, considerably large scale remodeling is necessary.
Either way, you'll need to write asm code.