Picture would help :)

Sorry I put it on there but it didn’t go through. I’ve attached it now

Is that a bark mulch on top or your soil? Looks very chunky. If it’s mulch, pull it away from your plant stems. If it’s what all your “soil” is like could be the problem.

Is it in a window box or perhaps a planter? Most tall garden phlox like Purple Flame are large plants and need a fair amount of space. It may well be the potting medium. What is it planted in such as the type of soil or planting medium and how deep is the container? How long has it been in that spot? How do you meet its water needs?

It’s in a window box but these aren’t large as they are a dwarf variety. It’s planted in soil with a layer of mulch on top. I have another window box and they are doing amazing. It’s just these. I thought it might be some kind of disease like yellow asters?

I agree with kaillean, looks like they are being smothered. Pull the mulch back and let them breath. Are you overwatering? Check the soil with your finger (stick it deep down) before you water. You can also remove the ugly foliage....

Small is relative. They are far shorter than the average garden phlox, but still expected to get to most of a foot across, which is larger than the typical window box can accommodate well.

I agree with others that water, either too much or too little at some point, is the most likely culprit. It is difficult to keep window boxes appropriately moist because it is usually such a small soil mass. If I ruled the world, window boxes would have to be at least 8” across and a foot deep. ;>)

Powdery mildew, which is the most common disease of phlox, typically doesn’t cause brown leaves this early in the season IME though I suppose it is possible.

These window boxes are about 3 feet wide and they are specially designed with grooves in the bottom to allow water to be absorbed as the frame is filled with water to decrease how much you have to water

I have two in two different beds. One was chlorotic as heck but doing better, the other, perfect. Just a few thoughts.... Were the plants healthy with good roots to begin with? As mentioned, could be too much water causing the issues. Maybe the planter is holding on to too much and not giving the plants a chance to dry down? Definately pull the mulch back from the base to avoid chances of rot. Could be a disease of some sort possibly. Here is a quick link of some that affect them. Aster yellows is a virus they can get and it is mentioned briefly in the link along with others.

https://www.extension.umn.edu/garden/diagnose/plant/annualperennial/phlox/leavesdiscolored.html