The problem identified with the Altura 2.5kg appears to be twofold: First, the foam filling of the hard shell has either shrunk or the shell was inadequately filled during manufacturer; Second, the plug is either missing from the hole through which the lifebuoy shell was filled or wasn’t there in the first place. The latter may not be immediately apparent because it is beneath reflective tape.
Check the lifebuoys on your vessel is there water inside? Is it heavier than it should be? Is the plug missing?
Lifebuoys are often overlooked in maintenance checks but they should be checked for integrity.