I found this article regarding a true full backup:
Force a Full Backup Out of Schedule
https://kb.msp360.com/managed-backup-service/troubleshooting/Forcing-a-full-backup-out-of-schedule

It looks like it can be triggered manually. Will an existing backup plan ever complete a true full backup if the administrator does not trigger this manually?

Hi Jim - Steve ( one of the few, like you, who actually pays attention to backup sh*t and participates in this forum.)
A few comments regarding your question(s):
The promise of 5-10% savings in Cloud storage space for 8.1 is enticing. But it requires a "True full" backup to gain the benefit.
We do HYPERV backup using Synthetic fulls - so we would need to do a "true Full" and upload everything again which we could do using the "force Full Backup" option..
But since we are utilizing Object Lock and GFS, in my mind it makes no sense to even attempt to change to what we have just to save 5-10% of the $6/TB.Mo we pay for BackBlaze.
But 8.1 introduces another issue that I reported to MSP360 today:
We set retention to 7 days, Weekly GFS to 1 week,and Monthly GFS to one month. This "1-1-1" policy minimizes our storage costs by keeping only Three Fulls at any one time while getting the benefit of immutability as required for passing the audits for our government/healthcare clients.
But the new version is flagging these settings as invalid, saying that [something] "cannot be less than or equal to the retention period but does not say what.
Waiting for a response from support to explain not only what the change actually means, but also why ir was changed.
.

Thanks Steve. This is very helpful information. I hope that the GFS settings issues is easily resolvable.