For what's it's worth...this is the result I get in Mainstage reset and rescan for both PanMan and SpaceBlender. All the other Soundtoys plug-ins work and pass validation in DP, Mainstage, and Garageband. In any event, it is what it is. Although, I do feel a bit like a beta tester with this experience. It is "Free" until May 22nd

Hmmm....
validating Audio Unit PanMan by Soundtoys:
AU Validation Tool
Version: 1.10.0
Copyright 2003-2019, Apple Inc. All Rights Reserved.
Specify -h (-help) for command options
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VALIDATING AUDIO UNIT: 'aufx' - 'PMN ' - 'SToy'
--------------------------------------------------
Manufacturer String: Soundtoys
AudioUnit Name: PanMan
Component Version: 5.4.3 (0x50403)
* * PASS
--------------------------------------------------
TESTING OPEN TIMES:
COLD:
Time to open AudioUnit: 469.501 ms
WARM:
Time to open AudioUnit: 0.058 ms
This AudioUnit is a version 2 implementation.
validation result: crashed validation
validating Audio Unit SpaceBlender by Soundtoys:
AU Validation Tool
Version: 1.10.0
Copyright 2003-2019, Apple Inc. All Rights Reserved.
Specify -h (-help) for command options
--------------------------------------------------
VALIDATING AUDIO UNIT: 'aumf' - 'SpBl' - 'SToy'
--------------------------------------------------
Manufacturer String: Soundtoys
AudioUnit Name: SpaceBlender
Component Version: 5.5.0 (0x50500)
* * PASS
--------------------------------------------------
TESTING OPEN TIMES:
COLD:
Time to open AudioUnit: 457.035 ms
WARM:
Time to open AudioUnit: 0.022 ms
This AudioUnit is a version 2 implementation.
validation result: crashed validation