After the LLD rules executes in, if the trigger title reaches over 256 characters, then LLD cannot create it and says:

Cannot update trigger: value "" too long

Real life example:

Cannot update trigger: value "/var/lib/lxc/core-abcdefg: Disk space is critically low (used > {$VFS.FS.PUSED.MAX.CRIT.LINUX:"/var/lib/lxc/core-abcdefg"}% and free < {$VFS.FS.FREE.MIN.CRIT.LINUX:"/var/lib/lxc/core-abcdefg"}, for more than {$VFS.FS.TIME.LINUX:"/var/lib/lxc/core-abcdefg"})" is too long.

If try to create the trigger manually (without LLD) then frontend automatically truncates the trigger title to 256 characters:

According to documentation, the trigger.description column can hold up to 2048 characters. So it looks like the database is not creating limitation, but process 'zabbix-server' or frontend.