Is the non-Spam email you want to not go into Spam folder, identifiable in any way?
Try going into GMail's
Settings (the little gear icon among the cluster of functions/icons at upper right in your GMail window)
then go to the
Filters and Blocked Addresses
section, then go to the lower half, labeled
The following email addresses are Blocked [etc.]
and look for your Thai frequent-correspondent's email address there. And select that address, if found there, and then when that Thai correspondent's email address is selected, click on
Unblock Selected Addresses
and see if that frees up the system to properly sort them into Inbox again.
(Also make sure the Thai correspondent's email, or emails, is not listed twice, and remove all instances if so.)
Let me know how that goes, by and by or at length, please.