Windows Server 2008 R2 Hyper-V guest (sometimes) does not get enough dynamic memory

I have a Windows Server 2008 R2 Hyper-V guest (the host is Windows Server 2016) which sometimes (for example, right now) fails to get more memory even though

1. dynamic memory is enabled,
2. plenty of memory is available on the host, and
3. the guest requires more memory.

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As soon as I manually restart the guest, dynamic memory allocation works properly again (until it stops working properly again).

I have found the following similar questions and resources, but they only apply to other operating system versions:

• Hyper-V dynamic memory not correctly assigning more memory (applies only to Windows Server 2012/2012R2)
• Hotfix: Hyper-V Dynamic Memory does not work on a Windows Server 2008 Standard Edition or Windows Web Server 2008 virtual machine (VM) (applies only to Windows Server 2008)

Is this a known issue? If yes, what can I do to fix it?

Note: Since the problem only occurs sporadically, I won't be able to give timely feedback on "Try this: ..." answers.