Qt 5.15 doesn't have prebuilt version for VS2017

VS 2017 and VS2019 are binary compatible which makes it possible to use Qt msvc_2017 with Visual Studio 2019. However, this doesn't work the other way around - applications compiled with VS 2017 and linked to Qt 5.15 (msvc_2019) silently crash at startup.
This is documented by Microsoft and they clearly say that you have to use the newest visual studio version among the libraries you're using:
You can mix binaries built by different versions of the toolset. However, you must use a toolset at least as recent as the most recent binary to link your app. Here's an example: you can link an app compiled using the 2017 toolset to a static library compiled using 2019, if they're linked using the 2019 toolset.

https://docs.microsoft.com/en-us/cpp/porting/binary-compat-2015-2017?view=vs-2019