How are the jobs set up as forever forward incremental?  It sounds like the one chain could be corrupted causing the issue but cannot say for sure with both being the same. I would try doing a full backup and then see from that point forward what happens.

Hi, do you have any mentions in the job run about VMs no longer being processed by the job? Veeam doesn’t automatically purge backups that are the latest backups of a VM, unless you have the automatic delete option set in the job, try that

Chris is right. I think the first option is to take a full backup of the problematic job and check it. The chain in the relevant job may be completely broken or it may not be able to detect something. You can create a new job and continue. If there is important data in the old job, it can be deleted by waiting until the RTO period ends.

Did you check the job config.
Did you configure something conflicting?
Maybe post the config ;)

As everyone told, this is probably happening because the job is not configured to run in the Forever Foward Incremental.
Go to Advanced Setting in the Storage section and check how it’s configured. It should be like this:

If it’s selected to create synthetic full or active full backup you must deselect those options.