Recently I encrypted my LG Tribute HD. I was doing something in recovery mode a couple days later, I accidentally hit the "wipe all data on phone" button. I pressed no. I rebooted my phone and all of the sudden secure startup was enabled and it had a password instead of a pattern. I tried a couple passwords but none of them worked. The attempts count only went down 1 time for some weird reason. I don't want to factory reset my phone. I just did a couple days ago, and I don't want to again.
Well I've ended in same place as you but with entering number in a dialer. White screen with only text box to enter password, and counter was going only to 29/30 tries left no matter how many times you enter password. This one? One user said that this happened after he tried to clear dalvik/cache from stock recovery on LG (I think it was about K10). My guess that this action wipes the encryption key so when you try to boot after clearing it prompts you this, and you probably need to enter key which I'm sure you don't know for this user and me too factory reset was only way out. See this and this and this
I've read on a couple of forums where people have had the issue of their phone randomly rebooting and starting up to a lock screen that reads (something like):
Secure startup 1/25 attempts
If you use a pattern code to unlock your phone, the PIN number will be the digits that your pattern lock follows.
I don't know how else to word it. Say, your pattern followed the corners to 7391, then your secure startup PIN would be 7536951. So, if you use a very complex pattern, it will just be every digit used that the pattern follows.
The attached visual reference would be 1598753.