IIS App Domain restarting unexpectedly

We have handler based web application hosted on IIS. It is "HttpHandler" based routing application. The handlers receive approximtalet two to three thousand requests per second to serve data. The IIS App_domain getting restarted unexpectedly without anything configured on iis level and system level. All the in-memory data lost on this process which we need to maintain user sessions.

The issue began about 4 months back and happens 2 to 3 times a week, but only once in a day
No errors in the IIS and event logs found.
We collected the crash dump (attached) which shows Culture not found exception

Environment
• OS - Windows server 2012 R2 standard
• IIS - IIS 8.5.1600.16384
• Framework - .NET Framework 4.8

We checked event viewer logs and IIS logs but nothing was written
We enabled all log options in IIS settings, but nothing gets logged
We did windows update to latest version
We upgraded the framework from 4.5 to 4.8
We converted the application from 32 bit to 64 bit (This was done because of out of memory exception)
We collected the crash dump (attached) which shows Culture not found exception