I know very little about reloading and your type of gun but in 50 degrees and colder my 30.06 xpr will keyhole 3 shot groups at a hundred yards consistently,as long as I do my part but at 70 degrees and hotter the first 2 are good but the 3rd is off by at least an inch. After that if I try to shoot before barrel cools completely it will open up to a 2 1/2 inch groups. This is with factory ammo. I don't know if heat is your issue but it is for this particular gun, good luck.

I would also check the rifle over pretty well. Wood stock? Maybe it swelled in the summer humidity? Check everything over and review your shooting technique (what is the forearm resting on?) before you go too deep in to the ammunition.

First thing I would suspect is copper fouling. Get out the ammonia based cleaner and soak it a bit.

just went through this with my gun. It started to shoot horrible cleaned the thing and now is back to normal.

A lot of good info above.
To answer your question, do good loads go bad?
No.
I regularly go back to recipes I developed years ago an duplicate them with results consistent to the original.
Switching lots on primers, powder, bullets can and sometimes will open groups a little. Sometimes they will get a little tighter. But I personally have never seen 1.25” groups turn into “all over the place”.
Go back to basics. You are missing something. Start with the rifle then the scope then the ammunition.

Single stage and hand weight all charges

Ar15 has a poly lower but wouldn't figure it would be a problem

Same lot on everything brass was range brass ( lake city 5.56 but made sure to pick all the same hard stamps )

you said it was still stacked your 53gr , what is your twist rate? you need a fairly fast twist for 75gr like 1:7

a 1:9 could put you right on the edge of stability and the difference that puts you off could be very minor if you were already on the edge of stability.

Thought in your original post you weren’t sure if the powder measure was off.
Maybe I misinterpreted.

I would try this. I would also put a box of loaded rounds in the freezer overnight and then transfer to the cooler on the way to the range.

Wolfie.........just so we are clear..you said when you originally fired this load it stacked 5 rounds in a quarter. Then when you fired again recently they impacted all over the place.

Were the rounds you fired most recently NEW handloads or part of the original batch loaded at the beginning ?

My gun is a 1:7 twist which from my reading shouldn't shoot those 53's as well as it does but it does.

Basically . When I was developing the load this perticular one was the best at 100yd and I was in a hurry so I just took it and cloned it. In all honesty I really should have done a few more tests. Tall the round I shot recently where new well new in the manner I shot up the original test loads then loaded these 50 and sat them on the shelf a few months ago
And these where grouping I around 4" @100yd. Where the originals where less than 3/4".
I thought it was me but then I let my buddy shoot who's a much better shot and groups about the same with him.

OK thanks. Sounds like maybe you have a glitch in the recipe. Check your scale and maybe drop down a couple grains on the powder. Make sure your scale is level

scope mounts and barrel would be a good place to start.

1:7 should stabilize a 75gr and be less than ideal for 53gr however those V-max tend to be a very concentric bullet that fly much faster form 22-250 than the 223 so spinning them faster twist but not higher speed probably doesn't effect them much.

I have found my 1:7 all shoot 55gr fairly well but I am not pushing velocity with anything.

I would chill some rounds down and see if that makes any difference easy test you already have rounds

the 8208xbr only has a 2.7gr range for start to max I would probably start over and load a 20.0, 20.5, 21 ,21.5 ,22 and look for a good group between 2 other good groups.

the other thing is and it costs money up front to shoot more bullets but it confirms the load , you can get lucky on a group especially of 3 but much harder to get lucky on groups of 5 harder yet on several groups of 5

I think if you shoot the test groups 10 at each of the 1/2 gr increments you will find a range where it is good , you can fine tune on that but in the middle of good has better practicality than the search for perfect.

few of us really need any better than MOA there was a long range competitor and instructor , who said you can search for sub MOA and it can be a perpetual quest but MOA will do. Better to have a load and rifle that is MOA all the time than one that is sub MOA when the stars align. spend your ammo learning your range and reading the wind than a quest for bench perfect condition sub MOA