I know that behind the scenes, it seems the chances of getting a prime engram do decrease after obtaining one in a given day. But I'm pretty sure that the visual Prime Attunement buff indicator that appears on the character page is always there? But I'll admit that I might just not have noticed that before (or at least since I reached max power)! Thanks for the reply, I'll see if it comes back tomorrow and hopefully it'll be there.

the buff goes away once you are caught up. You get 1 per day, if you do not play for 3 days, then you are behind, so the buff will stay there for the next 4 engrams you earn (1 for each missed day and 1 for the day you are currently on). Once you are back on the 1:1 it will leave after obtaining the first one. Likewise, if you somehow how get ahead on them you won't see the buff again until you make up the days.

Wow, there's a lot more going on with the Prime Engrams than I knew! Thank you very much for the information, it's really appreciated!