IMHO it's more perfidious than that. I think they simply don't want us to use the app anymore.
Even if you have your Play Store country and payment profile set to Thailand it still won't come up in a search.
This is the URL for the app that I extracted using the Play Store's share function. Clicking on it will likely bring up a blank screen instead of the usual "this app is not available in your region" message.
https://play.google.com/store/apps/details?id=th.go.immigration.section38
Also, the app itself hasn't been updated since May 28, 2020. I find this unusual as apps need to be updated every so often to stay current.
Finally, they are not advertising it on immi.go.th hence why I reckon it's a sunset application that they hope will fade into the distance ????