This is ringing some bells with me. There was an issue with MX450s related to single- and multi-threading. It is vague in my mind now. I also recall a related but different issue if it is being used as an AnyConnect concentrator. Is this MX450 also terminating client VPN? If you are not running a current stable or better firmware, do that first. I recall this issue was fixed. Also note that 19.2.2 and 19.2.4 fix quite a few MX450 issues. Personally, I think I would jump to 19.2.4. Otherwise (in desperation), ask support to see if there are any options they can see to disable multi-threading (as a test). There is a process they can control the behaviour of from their side. This is a low-probability "Hail Mary" type of option.
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