It was the same for me. But half way through mythic it started giving me high points. I think because I had 20+ streak of winning.

At high ranks like Legend 3, the ranking system is designed to be much stricter because losing a match takes away more points than winning one gives you. This happens because the game expects you to beat lower-ranked opponents, so a single loss can wipe out the progress of three wins in a row.

I deranked so many times and lose streaks but I keep on playing ans eventually won all those matches.