This has unfortunately been my experience as a player with Milestone in 5e. It didn't even feel like they were tied to progress or significant, well, milestones. Honestly, it felt like it was based on the DM either wanting to give us some excitement, or they felt like it had been "long enough." Now granted, that's just two specific DMs. If I were to use that system, I think I'd either want the players to know exactly what degree of progress would trigger it, or use the XP chunks as milestone as mentioned upthread.
But I'm happy with XP right now. I've never seen any XP chasing behavior from my players, I like seeing the number tick up closer to the known point at the end of each session, and I'm also a fan of being able to highlight each individual success, no matter how small it may be.