We recently upgraded a site from CMS 11 to CMS 12 and the site is up and running fine after a few bugs. Now, a few months after the upgrade and deployment there is another bug that I can't quite wrap my head around and that seemingly has something to do with forms. It causes the w3wp to crash and the site to restart.

Looking in the production server's Event Viewer, I get the following error:

Application: w3wp.exe
CoreCLR Version: 6.0.1122.52304
.NET Version: 6.0.11
Description: The process was terminated due to an unhandled exception.
Exception Info: System.ArgumentNullException: Value cannot be null. (Parameter 'source')
   at System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
   at System.Linq.Enumerable.Count[TSource](IEnumerable`1 source)
   at EPiServer.Forms.Core.Internal.DataSubmissionService.IsMalFormSteps(IForm form)
   at EPiServer.Forms.Core.Internal.DataSubmissionService.GetSubmittableStatus(IForm form, HttpContext context)
   at AspNetCoreGeneratedDocument.FormsViews_Views_ElementBlocks_FormContainerInitScript.ExecuteAsync()
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage page, ViewContext context)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, ViewContext context, Boolean invokeViewStarts)
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(ViewContext context)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.HtmlHelper.RenderPartialCoreAsync(String partialViewName, Object model, ViewDataDictionary viewData, TextWriter writer)
   at Microsoft.AspNetCore.Mvc.ViewFeatures.HtmlHelper.PartialAsync(String partialViewName, Object model, ViewDataDictionary viewData)
   at AspNetCoreGeneratedDocument.FormsViews_Views_ElementBlocks_Components_FormContainerBlock_FormContainerBlock.<>c__DisplayClass0_0.<<ExecuteAsync>g__RenderFormBody|0>d.MoveNext()
--- End of stack trace from previous location ---
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_1(Object state)
   at System.Threading.QueueUserWorkItemCallback.<>c.<.cctor>b__6_0(QueueUserWorkItemCallback quwi)
   at System.Threading.ExecutionContext.RunForThreadPoolUnsafe[TState](ExecutionContext executionContext, Action`1 callback, TState& state)
   at System.Threading.QueueUserWorkItemCallback.Execute()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()

I've done some research on the internal methods it references (GetSubmittableStatus, IsMalFormStep) and the only idea I can come up with is that there is some sort of corrupt form post upgrade causing these internals to crash rather than return true/false as intended.

Looking for any insight or ideas on what might be the cause or ideas of a solution.