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What version of PowerCLI are you using? I just tested with 6.5.4, and it definitely is not case sensitive.
Secondary question, how are you doing this checking? I specifically went and used the -Name parameter on both Get-Tag and Get-TagCategory... and both returned regardless of case on the string.
Marcel, thanks for the heads up. I have filed a bug to get this issue resolved.
Adding to Steve's comments, we found out that there is a vCenter version dependency with this issue. vCenter 6.5 is the one experiencing the issue, vCenter 6.0 behaves as expected.
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