Originally by @Legioth

The sync id that was introduced in #12337 can cause problems when testing using pre-recorded requests, e.g. with JMeter. To make it slightly easier to create stable requests, the server should ignore the sync id if the value in the request is -1.

It should also be possible to configure the server to ignore the value from the client so that already recorded requests would still work. This option could also make new requests use -1 to keep things stable even if recording new requests with some changes in the middle of the request sequence.