Indeed, i fill my 300 BAR 12 L bottle to the + side of 300 BAR, if i then walk away or use the bottle to fill a 300 BAR rifle the day later, well my 300 BAR have cooled down to a lot less pressure, but as privateer say it is quite normal.
If you get a bottle filled in a dive shop i have seen the very same, and even worse as at least here shops are reluctant to fill bottles to 300 BAR.
But as Privateer say, all you can do is to pump the bottle again once it have gotten colder, at least filling ( 180 - 300 BAR ) you can literally feel the 12 L bottle is hot, not cooking of course but no doubt it is hotter than ambient.
After i gotten my own big boy compressor i have to admit the times i have let the bottle cool down and then boosted it again to get it looking more like 300 BAR, well i think that can be counted on 1 maybe 2 fingers.
This might change with my new carbon fiber bottle, as it is after all 3 Liters less volume, but then again being like that it is also faster to top up to the 300 BAR.