回复：LG父母当时按时从美国出境去加拿大后才回中国，

They should surrender their I-94 forms to the US airport staff before boarding the airplane or to the Canadian border police if they entered Canada by land. If so, no problem in general.
If not, they should correct this problem by sending their I-94 and other supporting documents to USCIS. Otherwise, US government has no record of their leave. They are still considered staying in US.
Good luck!