Charging-only mode is part of Android, so the phone must be able to boot the high-level OS to charge. When you connect a power source, it will always start the Android boot process. If the off-mode-charge flag is set, it will start the charger program instead of the full Android UI.
Yep, you triggered the "battery protection bug". The problem is in the closed-source firmware, and relates to the PMC configuration. LineageOS doesn't have licenses for this source code, so we need F(x)tec to fix it. The only way to avoid it is to periodically charge your Pro1 X, and once a month should be