Encryption password has 2 digits

[AC Question]

I use an app that switches my screen lock PIN between 2 digits (when I'm connected to my home WiFi) and a much longer one (when I'm away from home). This morning I decided to switch on disk encryption. This went smoothly until the final reboot. As I am home, I expected the 2-digit PIN to work. It doesn't. Nor does the longer one. Is it possible that a valid screen lock PIN (that I've been using for years) would be rejected by the disk encryption UI, for being too short? Is there anything else I should try before letting it wipe my phone?

My phone is a Sony Z3 Compact.