You’ve run into a weird SmartThings limitation (bug?) that affects all external OAuth SmartApps (including IFTTT, The Ubi, SmartTiles, etc.): only one install is permitted per SmartThings Account, rather than per Hub or per Location (the Location selection drop down box is only applicable / functional, for the first attempt to install or if that instance is completely uninstalled).
This is due to way that references to the shared SmartApp object are handled internally by the SmartThings platform. It doesn’t generate unique install references at the Location level.
Please raise this up as an issue to Support@SmartThings.com, but I please use IFTTT as the example, because SmartTiles is not supported by SmartThings. To replicate the problem with IFTTT you need to create a second IFTTT Account and attempt to link it to your second Location/Hub. You’ll encounter the same problem.
There’s an existing Topic thread on this:
Version 6 of SmartTiles (under development, no release date published) will likely implement a workaround for this problem if SmartThings hasn’t fixed it by then. Support for multiple Locations is on the planned feature list and we have some ideas of how to do it. (Multiple Locations/Hubs per Account are relatively rare, so this is a lower priority than other features.)