I'm surprised Greg didn't start off with the usual "Make sure to mixdown to the same sample rate that was used during recording" as I think that more closely addresses this issue than it does on any of the posts where that is his comment. Anyway, I suggest you check the bottom of the interface before each mixdown to see what sample rate is being used by Mixcraft as audio interfaces will sometimes default to different settings than you are used to using. (For example, I have a problem whenever I record the computer screen output using Videomeld as it will only record the sound if I set mixcraft to use 'Core Audio' as the sound sub-system, even though I usually use the same audio output device in 'ASIO' mode. If I forget to reset the sound device to use ASIO mode after I've recorded the screen I have problems occurring. So my point is check the sound sub-system etc to make sure that the right sub-system [listed in Preferences, Sound Device] is being used by Mixcraft and the sample rate etc is the one you were it expecting it to be).
Note - it would be helpful if 'Core Audio','ASIO' & 'Wave' were listed by Mixcraft as 'Sound Sub-Systems' as this would reduce confusion in the mind of the user who believe that the Sound System is the name of their Audio Interface device
Mixcraft 8 Pro Studio Build 418 (64bit)
AMD Ryzen Core 3.0 GHz,
2TB SSD etc, 40GB RAM,
Windows 10 Professional
Focusrite Scarlett 18i8 (1st Gen, USB2).