HMACSHA256 causes AccessViolationException when reused parallelly

Question

Why there is nowhere written that HMACSHA256 can cause System.AccessViolationException when accessed parallelly? The AccessViolationException is super hard to investigate since it cannot be caught by regular try-catch (from .Net 6.0 it cannot be caught at all) and since it is usually thrown elsewhere than where it was caused.

Or even more concerning: why nowhere in documentation is written that (HMAC)SHA256 shall not be reused at all? See example of same strings being hashed differently.

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Why I am asking:

Our newly written app was randomly crashing after minutes or hours. In the Windows Event Viewer we then found:

Faulting application name: XXXXX.exe, version: 1.0.0.0, time stamp: 0x62571213
Faulting module name: coreclr.dll, version: 6.0.522.21309, time stamp: 0x625708f4
Exception code: 0xc0000005

Exception code 0xc0000005 is the code of the AccessViolationException. It took days to determine what was causing the problem and we were rather lucky to identify it: since we do a lot of optimization it seemed reasonable to cache the instance of HMACSHA256 and reuse it. Eventually it was accessed by two threads at once and caused the application to crash without any log message (there was only the error code in the Event Viewer, no stacktrace).

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Code example producing System.AccessViolationException immediately:

var keyString = "abcdefghijklmnopqrstuvwxyz0123456789";
var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(keyString));

var parallelOptions = new ParallelOptions() { MaxDegreeOfParallelism = 4 };

static IEnumerable<string> EnumerateStrings()
{
    while (true)
    {
        yield return "A short string";
        yield return "Lorem ipsum dolor sit amet, consectetur adipiscing elit ...";
    }
}

Parallel.ForEach(EnumerateStrings(),
                 parallelOptions,
                 theString => hmac.ComputeHash(Encoding.UTF8.GetBytes(theString)));

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Disclaimer: I do not require an answer to this question. I just wish somebody has asked exactly this question somewhen before me - it could have given me some clue. I would probably spend less time hitting my head against the wall. Hope it will spare some other heads.