Yes, in fact that has been a problem in the past (not to that extreme) because Calyx used to set the SPL to the actual level they shipped, which lead to people not being able to go back to stock because FPOS is always lagging behind.
That’s why they started setting it to the latest FPOS SPL, even if CalyxOS is ahead.
The problem is that according to the docs the rollback_index should actually be reset when installing a new ROM, but so far no one has been able to explain why it isn’t. FP isn’t very communicative in that regard, and the Calyx devs didn’t get an answer out of them either 
Not exactly, the rollback_index used is in this case just the unix timestamp of the platform security update. It’s set in BoardConfig.mk, so you could just use a different int if you wanted, as long as you increase the number.
Here’s how it looks for CalyxOS …
BOARD_AVB_VBMETA_SYSTEM_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
[…]
# Security patch level
BOOT_SECURITY_PATCH := 2023-04-05
VENDOR_SECURITY_PATCH := 2023-04-05
# Calyx
PLATFORM_SECURITY_PATCH := 2023-04-05
STOCK_SECURITY_PATCH := 2023-04-05 # Latest stock update available
PLATFORM_SECURITY_PATCH_TIMESTAMP := $(shell date -d 'TZ="GMT" $(STOCK_SECURITY_PATCH)' +%s)
… as you can see, it gets set to the stock SPL.
I wrote a bit more about it here, I’m not a dev though, beware off potential errors.