The plastic failed
Updated
On this page8
What it looks like
A cap that has gone tacky or soft, a reducer swollen tight in the neck, a liner that has lifted or gone cloudy, or a cloudy ring in the liquid where something has dissolved into it. None of it shows on the filling day.
Why this category in particular
Citrus terpenes, eugenol, cinnamic aldehyde and several other common fragrance materials are aggressive solvents. The blend that smells best is quite often the one hardest on a closure, which is not a coincidence you can design away.
It is a compliance failure too
If the package is special packaging, 1700.15(a) requires it to keep functioning in actual contact with the substance. A closure that has softened has stopped meeting the standard, whatever the datasheet said when you bought it.
Liners are the quiet one
A cap can be sound while its liner is not, and the liner is the part touching the liquid. Check what the liner is made of and test the assembled closure rather than the shell.
Glass removes the bottle from the question
Which is why all 63 containers on this shelf are glass. It does not remove the closure, the liner or the reducer, so the plastic you still have is the plastic you have to test.
Test by waiting
Fill, cap, stand it upright and lay a second one on its side, then look at both at one, three and six months. Every failure above is visible by then and none of them is visible on day one.
Choose the liner deliberately, not by default
Caps are usually quoted with whatever liner the supplier stocks, and that is the part most likely to fail against a fragrance blend. Asking what the liner is, and asking in writing, is the single most useful question to put to a closure supplier in this category.