I'm not already familiar with a condition that would cause the credential provider to randomly or consistently not appear in the presence of Bitlocker.  Probably the best first clue to review would be the NCCredProvider log from a current SP7 (IR4) client, since there was some additional logging added in that latest version which might provide additional context.

You enable the logging with a DWORD value named "Debug" set to 0x3 under [HKEY_LOCAL_MACHINE\Software\Novell\Authentication\NCCredProvider] (How to collect an NCCredProvider log (microfocus.com)), and the log files are written to C:\ProgramData\Novell\Client\Log\.  Note you don't have to unhide this folder, and can just enter "C:\ProgramData" in the File Explorer address bar and it will let you browse the still-hidden folder.

What we might see in the NCCredProvider log is simply "nothing", meaning we were called as an ICredentialProviderFilter but then no further activity.  Which would be evidence that some other credential provider filter had filtered us out.  NCCredProvider's own log can't really reveal the reasons "why" some other credential provider chose to filter us out, though.

Or, maybe we'll see some failure of initialization occurring for NCCredProvider itself, somehow indirectly related to Bitlocker being enabled.  Nothing seems particularly logical or "likely" about that, but certainly this would explain the perhaps "random" results and give us clues to follow.

edit: I realize I should have explicitly added: If you have a device exhibiting the "random" behavior, capturing both a log from a time when the failure occurred, and a separate log showing what happened when the failure didn't occur, would be a useful comparison.  Or if both symptoms were captured within the same log, knowing the clock time of the local computer for approximately when each result occurred, just in case the logged information isn't already obvious for there having been an issue.

The problem was just a missing registry value.

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System

DisableAutomaticRestartSignOn = 1

https://support.microfocus.com/kb/doc.php?id=7021904

Thanks to the support team