KNOWN_BUGS: restore text regarding #2101.

This was added earlier but appears to have been removed accidentally.

AFAICT this is very much still an issue.

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I say "accidentally" because the text seems to have harmlessly snuck
into [1] (which makes no mention of it).  [1] was later reverted for
unspecified reasons in [2], presumably because the mentioned issue was
fixed or invalid.

[1] de9fac00
[2] 16d1f369

Closes #2618