I had a message on my mobile banking app from my bank that with effect August 20th the site will only be accessible on the device last logged in with.

 

Any idea what that's about?

No idea, but I suppose you could uninstall and reinstall, thus always making your mobile device the device last logged in with.   

One of the security features SCB, TMB, LH Bank and KBank requires assignment of the app to a specific phone (must give the bank phone number). Bangkok Bank app is accessible on any phone with the app.

I can of course just speculate, if you want to know it for sure you should call your bank, but probably previously it was possible to have the banking app installed on multiple devices (phones, tablet...) and use them to access your bank account. They now limit this to only one device.

12 minutes ago, asiaexpat said:

One of the security features SCB, TMB, LH Bank and KBank requires assignment of the app to a specific phone (must give the bank phone number). Bangkok Bank app is accessible on any phone with the app.

You can take a sim card out and put it in another device.  

 

It would have to be the MAC address they lock onto.  

Banks are major hacking targets, and it looks like this is another step they are taking to make the hack more challenging for the bad guys. Unfortunately, while it keeps our money safer, it also makes banking more inconvenient.

"More than 25% of global malware attacks hit financial service providers."  https://11fs.com/blog/how-are-banks-dealing-with-a-rise-in-cyber-attacks