SAFETY and INTELLIGENCE can confuse some folks.
"Checked OK when filled - but leaked later" may be a sign the bottle was overfilled.
Filling by exact weight is VITAL. Eyeball to bottle I have seen overfilled bottles BULGE. Note the plural conjugation of bottle. The relief valves did not pop. Bulge so bad they wobbled.
Even a digital bathroom scale can be used if empty bottle "tare weight" is Factored. Wear gloves. Vented liquid WILL cause frostbite.
A forced heavy liquid vent (bottle upside down) cleared many leaking ports for me.
This was done down here where dozens of campers showed up on a remote beach. 1 lb bottles were not available anywhere. One guy showed up with 30+ bottles from another camping beach 35 miles away. The were desperate for lantern light.
I not only froze the bottles I left the 20 lb pot out in the sun. Put a 1/32" restriction orifice with valve inline with fill hose. Charged a buck a fill and my man Rodrigo made a fortune at eqvt 50 cents a bottle while I snoozed in a hammock. Too many bottles refused to seal. Two blowdowns and it failed the test. I smeared lanolin onto the bottle threads.