Yes, very strange.
And I have tested on many os (Linux, FreeBSD, WinXP, Win7, win8, win81,win10).
And, indeed with win10 there are synchro-problems sometimes.
But I have also problems with all other audio-applications (that are not win10 native).
I also have problems with audio-drivers win10/64 bit.
It would be great if you give in attachment a demo with old revision (working for you).
In SimpleDrums demo code, it uses uos_AddFromFile().
But you may try with uos_AddFromMemoryStream(), the code is there ready to use.
You only have to uncomment uos_AddFromMemoryStream in demo code (and comment code using uos_AddFromFile).