Crash when using code quotations in tests

We do our unit tests using [the unquote library](https://github.com/SwensenSoftware/unquote) to write our assertions. This relies a lot on F# code quotations. Most of the time our tests run just fine but once in a while a test fails with a NullReferenceException. The weird thing is that both the test and the code under test are pure so I find it hard to understand where the flaky behavior comes from.

I've [also reported the bug to unquote](https://github.com/SwensenSoftware/unquote/issues/160), but unfortunately the author also doesn't know. Because the stack trace only contains `FSharp.Core` calls I was hoping maybe someone here might have an idea:

```
System.NullReferenceException : Object reference not set to an instance of an object.
Stack Trace:
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(112,0): at Microsoft.FSharp.Quotations.FSharpVar..ctor(String name, Type typ, FSharpOption`1 isMutable)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2070,0): at Microsoft.FSharp.Quotations.PatternsModule.u_VarDecl@2070.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2048,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2048-2.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\local.fs(236,0): at Microsoft.FSharp.Primitives.Basics.List.mapToFreshConsTail[a,b](FSharpList`1 cons, FSharpFunc`2 f, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\local.fs(246,0): at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2031,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2030.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\local.fs(245,0): at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2031,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2030.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\local.fs(245,0): at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2031,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2030.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2059,0): at Microsoft.FSharp.Quotations.PatternsModule.u_Expr@2059-5.Invoke(BindingEnv env)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2568,0): at Microsoft.FSharp.Quotations.PatternsModule.deserialize(Type localAssembly, Type[] referencedTypeDefs, Type[] spliceTypes, FSharpExpr[] spliceExprs, Byte[] bytes)
D:\a\_work\1\s\src\FSharp.Core\quotations.fs(2781,0): at Microsoft.FSharp.Quotations.FSharpExpr.Deserialize40(Type qualifyingType, Type[] referencedTypes, Type[] spliceTypes, FSharpExpr[] spliceExprs, Byte[] bytes)
at Tomra.Ts.Core.MapTests.Keys works as expected()

```

This is the test:

```F#
let [] ``Keys works as expected``() =
let map = Map.ofList [ (1, "One"); (2, "Two"); (3, "Three") ]
test <@ map |> Map.keys |> List.ofSeq = [1; 2; 3] @>
```
And this is the code being tested:

```F#
namespace global

open System

[]
module Map =

let keys (map: Map<_, _>) =
seq {
for KeyValue(key, _) in map do
yield key
}
```

**Related information**

I'm using .NET SDK 6.0.300 and originally experienced the problem when using F# 5. This week we were finally able to switch to F# 6 and I was secretly hoping this might fix the problem but unfortunately it did not.