Error list math: Build + Intellisense < Build

_This issue has been moved from [a ticket on Developer Community](https://developercommunity.visualstudio.com/t/Error-list-math:-Build--Intellisense--/10246361)._

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[severity:It's more difficult to complete my work]
When building a solution, the error list includes warnings. After a few seconds (2-8 seconds), the list is truncated and important warnings disappear (that result in run-time exceptions). This is when the setting is Build + Intellisense. I am not expecting Build + Intellisense to ever contain less warnings and errors than Build alone, nor for the list to suddenly shrink after a rebuild has completed.

This particular issue, in combination with another error in WarningsAsErrors not working, resulted in shipping a critical defect in our software which should have been trivially caught by static analysis.

The solution consists mostly of F# projects and two C# projects.

The FS0025 warning disappeared from the error list.

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### Addition from comments (i.e error repro)
I should be more precise:

Error panel: Build + Intellisense
Start rebuild

Observe: error list contains FS0025 warning
Wait a few seconds
Observe: suddenly error list does not contain FS0025 warning
The issue does not occur when the error list panel is set to the Build message setting (i.e. without Intellisense). Since we manually audit the error (warnings) list before releases, we would have caught this warning, even if WarningsAsErrors did not help us in the first place; however since the warning vanished from view, even manual auditing for critical warnings did not help.