@John Thanks for posting in our Q&A.
Yes, it is by design. If you set the minimum PIN to 4 digits, it only pops up a page to set a PIN that is 4 digits, and we can't set a PIN that is 6 digits.
If you feel that the description of this setting is unreasonable, it is suggested to post in intune feedback portal. It is a place to collect customers' requirements, problems and doubts. Here is the link:
https://feedbackportal.microsoft.com/feedback/forum/ef1d6d38-fd1b-ec11-b6e7-0022481f8472
Thanks for your understanding.
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