Powershell variable treated as NULL in New-GlobalAddressList cmdlet

I can't wrap my head around what's going on here.
This works from the powershell commandline:

New-GlobalAddressList -name ThisGuy_GAL -recipientFilter {CustomAttribute10 -eq ThisGuy}

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Get-globalAddressList Thisguy_GAL returns the expected recipient filter that looks like this:
CustomAttribute10 -eq 'ThisGuy'


However, if I do the exact same thing in a script, I get a malformed recipient filter

$uniqueID = "ThatGuy"
New-GlobalAddressList -Name "$($uniqueID)_GAL" -RecipientFilter {CustomAttribute10 -eq $uniqueID}

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The above executes with no errors, BUT - Get-GlobalAddressList ThatGuy_GAL returns a recipient filter that looks like this:

-not(CustomAttribute10 -ne $null)

What?