Can you maybe check the visitor log or the day archive for 31-12-2018 whether the Thailand_2 pageview was from there?

It's hard to say why they were missing. Are they under the same folder? Or maybe shown somewhere else? Are they shown when you don't search? I suppose you archive data through the CLI / cronjob?

If you have many different page URLs (which could explain this issue), check out https://matomo.org/faq/how-to/faq_54/#faq_54 and maybe increase the limits. For example if you had heaps of different page URLs in December, Thailand_2 was possibly simply not included because it was not in top #5000 for example and it was grouped under #other. Same could be for Thailand_3.

You could try to increase those limits in your config file and then invalidate the data for December and January and archive them again which may fix the issue: https://matomo.org/faq/how-to/faq_155/