I have had the same issues running Finale on a Mac.  Scores with large varieties of percussion instruments that played under earlier versions of Finale drop voices under 25+ and demand maps that were not required before.  My experience with Make Music Tech support has not been very good.  I have usually had so find fixes on my own, else be subjected to repetitive "clean re-installs" without any change in the situations.  I think-- as David Eastmond's comments and those of others suggest, MakeMusic owes its users--everyone, but particularly those of us who have stuck with them for over a decade--paying good money for their products and those of Gary Garritan--a higher level of problem solving on issues such as the percussion layout (just one of many annoying problems).

Finale 25.5.0.259 in a Mac environment under OSX 10.13.6; 4.2Ghz Core i7,  32 GB 2400 MHz DDR4.  In terms of voices, I have CFX Lite, Finale Default Bank, Garritan Harps, GPO, Personal Orchestra 5, World Instruments (as well as a variety of other voices using Kontakt 6, UVI and Arturia).  I add this description, as occasionally commenters have suggested that more computing power would solve problems.  The set up I now use is far more bang than MakeMusic says is necessary.  I will be the first to admit that some problems -- such as the inability to stop playing (with related crashed) disappeared when I purchased the current (expensive) computer set up.  However, others -- such as those David F. well described -- are simply endemic to the Finale program current iteration.

I also hope some of the programers from MakeMusic read these comments and decide to fix Finale's impediments to composing.  I think the program as gotten away from its original intention -- which was a program for composers -- and become, instead, a program for music engravers.