Buried deep in the BIAB defaults is the solution.
When you select a style, at the bottom of the screen is a button labeled ACTION. One of the sub-menus is Set Dialog Options.
There are two check boxes that probably should be unchecked - Always Change Tempo to Best Tempo... and Change 4-bar Preview Tempo to best tempo for style.

It looks like these will honour the songs tempo rather than the style tempo.


BIAB for Windows Version 2019 (643)
Windows 10 (1909)