This happens randomly when I enter houses. One Sim will have this flowers particles around them as if they used the perfume, and it will not go away.
The last mods I downloaded are furniture and small game fixes such as better interests, and I have no idea if one of them are causing this issue. I tried resetting him, but the flowers are coming back.

Were they wearing perfume? Effects can get stuck if the Sim gets resets. Batbox has option to fix it, forgot name.

No perfume was purchased, it appears even on test hoods. FFS lot debugger seem to help, but this effect always returns when loading the lot again.

The only way to know if a mod is causing it (something this weird is almost always a mod thing) is to test it. Back up your game and troubleshoot. If just removing the most recent mods doesn't do it, you'll have to do the 50/50. It's tedious but it doesn't take as long as you think it will, because the game loads so fast without all your mods in.

What if you use perfume normally and let it expire? That should clear the perfume status.

It is possible that the effect was started by a mod independently of actual perfume. After you let the perfume expire, you could sort your mods by the most recent date and move some out to a directory off the Sims path to disable them until the problem disappears. Possibly, you need to expire the perfume again if a mod gave you a long duration. Then add them back in groups.

You say it is present in test hoods. So is it more than one person? You made those test hoods before installing the recent mods. Then it could be an older mod, and the list of candidates widens.

Note that you'll need to restore the backup even if never save - invalid tokens are removed at neighbourhood level

At first I thought it might be a strange glitch from Story Progression and didn't pay much attention to it, and now I'm pretty far behind to guess the mod that is causing this.
The conflict detector doesn't seem to conflict with anything related to hygiene or something in the same category. And I don't seem to find any mod about laundry or cleaning except for Supplies Mod, but that mod was installed before this glitch, so I'm pretty much at a lost.

Found a bandage for the problem using an invisible flower FX from this mod default replacement . No more flowers thank Goodness...

That's what the 50/50 is for. No guesswork, just the slog, and in the end, you have the answer. In the end, it's almost always faster and less frustrating than guesswork.

It's entirely possible that this is a side-effect of two mods or even something about your individual computer and game set up interacting in unpredictable ways, and when I say "unpredictable" I mean that literally. Programmers are fond of saying that everything the code does is perfectly logical, but what they don't say is, that since code -- particularly this code, which is extremely complex even before you start modding it -- is complicated and has not one single foot in the real world, from a human point of view, machine logic often might as well be chaos magic. At the human level, nothing code does is constrained to make any sense, no matter how inevitable and logical it is at the machine level.

I once had a blouse that caused the photobooth to crash when it was used to take a picture.

I, and at least one other person, had a problem with cyjon's Slower LTA Gain that caused the vacation clock to freeze after an hour. Cyjon remarked that this was like all the toilets overflowing when you flipped the light switch -- it was that bizarre, and to add insult to injury, it didn't affect most people. Something about the way we had our computers or games set up, our particular combinations of mods, or something else particular to us was breaking an otherwise perfectly sound mod.

Do the 50/50. It's the only way to be sure.

I'll do the 50/50 if anything else weird happens. The FX was the only issue, and hiding that FX was the first thing I thought about trying.