Based on the pattern you describe, temporarily disabling FileVault is a reasonable diagnostic test, but I have not found Apple documentation identifying FileVault as the cause of an incorrect clock at startup.
The important detail is that the incorrect time appears only before you log in after a cold boot. Once macOS is running, the time immediately becomes correct and remains correct.
That distinction is useful because the startup process before authentication is not identical to the fully running macOS environment. Apple documents that, on a Mac with Apple silicon, FileVault credentials are required during the boot process. Apple also documents that the Mac has a sequence of boot components that run before macOS itself is fully running. (Apple Support)
Apple separately documents that macOS can set the date and time automatically using network time services. (Apple Support) However, I have not found Apple documentation stating that FileVault controls the clock or that FileVault is responsible for an incorrect time in the pre-login environment.
For that reason, disabling FileVault should be viewed as a way of testing whether changing the startup and authentication path affects the symptom. It does not, by itself, test or establish that FileVault is the cause.
Before changing anything, I would first make the comparison as controlled as possible. After the Mac has been completely shut down for several hours, perform a cold boot and record the displayed time at the login/FileVault screen. Then record it immediately after authentication and again shortly after the macOS desktop appears. This establishes exactly when the correction occurs.
It is also useful to compare three situations: a cold boot after a complete shutdown, a normal Restart, and waking the Mac from sleep. If the incorrect time occurs only after a complete shutdown, that would provide useful evidence that the symptom is associated specifically with that startup path.
Network availability before authentication is another relevant observation. Since automatic time setting can depend on network time services, it would be useful to note whether the Mac has an active network connection at the pre-login screen. I would not change the configured time server at this stage; first establish whether the existing configuration behaves differently depending on the startup method. (Apple Support)
The FileVault test can then be performed if you want to continue isolating the cause. With FileVault enabled, perform several comparable cold boots and record the result. If you subsequently disable FileVault, repeat the same cold-boot test under otherwise similar conditions. If the symptom consistently disappears only when FileVault is disabled, that would establish a relationship between the changed startup configuration and the symptom. It still would not establish that FileVault itself is responsible for setting the wrong time.
One other point is important when interpreting that experiment: disabling FileVault does not mean that an Apple-silicon Mac simply becomes an unencrypted computer. Apple documents that internal storage on Apple-silicon Macs uses hardware-assisted encryption and that FileVault provides an additional user-authentication layer for protecting the volume. (Apple Support) Nevertheless, changing FileVault is a security-related configuration change, so I would use it only as a deliberate diagnostic test rather than as a routine troubleshooting step.
At this point, I would therefore describe the situation as follows: the symptom is clearly associated with the pre-login portion of a cold boot, but the available Apple documentation does not establish what component is responsible for the incorrect time. The most useful next step is to determine whether the behaviour changes with cold boot versus Restart versus wake from sleep, and then, if necessary, use the FileVault change as a controlled diagnostic comparison.
I have not found Apple documentation describing this exact pattern—an incorrect clock only in the pre-login environment following a cold boot, followed by an immediate correction once macOS is running—as a documented limitation or known issue.
Relevant Apple documentation:
Boot process for a Mac with Apple silicon
Volume encryption with FileVault in macOS
Startup security in macOS
If you can't change the time or time zone on your Apple device
Intro to FileVault
