Sage 2018 - upgraded to 2019 and then downgraded back to 2018, now API won't work

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We have written a program for one of our clients using the Sage Peachtree API. Every time they upgrade versions, we always have to get the latest DLLs and rebuild our app to make it work again. However recently they say that they installed Sage 2019, ran into some problems, and then uninstalled and reinstalled their 2018.

Our current app version is compiled with relationships on Sage.Peachtree.API.dll version 2018.1.0.131. And that is the DLL on their machine, and it finds it just fine. However we get the error message:

Could not load file or assembly 'Sage.Peachtree.Domain, Version=2018.2.0.197, Culture=neutral, PublicKeyToken=d06c16dde04d83e4' or one of its dependencies. The system cannot find the file specified.

In checking, their Sage.Peachtree.Domain.DLL file is 2018.1.0.131, as would be expected. However it is clearly trying to find a different version (and failing). I'm guessing the version it can't find is the 2019 version.

Does anyone know why it is still looking for this newer version of the DLL, instead of using the version that's installed (and matches the API DLL)?