Export to wav, sometimes doesn't sound right using Timbres of Heaven

MuseScore 2.03, windows10
I sometimes use the Timbres of Heaven (ToH) soundfont for the string section (Violin I, Violin II, Viola, 'cello, contrabass). The other instruments (and vocals) are the default FluidR3mono.
On some of my scores (but not all), when exported as wav files, the playback of the wav is very different than playback straight from the MuseScore score. Especially, the string section is sometimes missing (or much reduced in volume).
I did not experience this problem in earlier versions of MuseScore; the wav always sounded identical to the score no matter what soundfonts were being used.
The four attached scores are well-balanced using only FluidR3, and if you export them to wav they should sound identical.
>>To demonstrate the problem, change the string sections to ToH (Don's Strings, Pizzicato Section, Tremolo Strings). You may want to adjust the mixer settings to rebalance the overall sound. I typically dial the mixer settings down by about 3 "hours" (reading the mixer dial as a clock face) when using ToH.
>>Then re-export to wav (I recommend changing the name so as not to replace the previous wav files).
Hopefully the problem repeats, and you will hear the same gross imbalance that I experience:
--In Carmen- Votre Toast, the strings are missing or so soft as to be inaudible.
--In O mio babbino, the strings sound fine when the piece opens, but when the harp & the voice come in, they are way too loud.
--In the Sextet from Lucia, the opening pizzicato in the strings is too soft relative to the voices.
--On the other hand, the Don Giovanni piece does *not* have this problem. A wav export sounds fine no matter whether the strings are ToH or Fluid.

In case it's important, in my Synthesizer settings Fluid is listed above ToH.

Thanks in advance for your help,