The gold remained inverted throughout, with the viewer needing to consider two points, one implied, the second specified within the script.
The gold was still inverted when it was 'thrown' to The Protagonist. He appears to have deliberately mishandled it, believing that he has not yet had his cover blown to reveal his real role as a member of Tenet. When we first see TP get used to handling inverted objects at the lab, he appears to quickly master the techniques involved. 'Instinct' is how it is described. However, in the scene on the boat prior to the Talinn chase he uses not just one but two opportunities to pretend to be ignorant of both inversion and the clandestine world he has been exposed to. Not only does to he appear to mishandle the inverted gold, he also refuses to acknowledge the Opera pass phrase with what should have been 'and there are no friends at dusk'
SATOR:They’re usually buying, not selling. But we do live in a twilight world...
PROTAGONIST: Is that Whitman? It’s pretty.
Finally, with regards to why The Protagonist deliberately fumbled the handling of the gold bar the script specifies that he used this opportunity to grab a soil sample for Neil's analysis...
The Protagonist, looking Volkov in the eye, picks up the gold bar, scooping DRIED MUD into his hand, unseen.
...which confirms that the bar is inverted and from the future.
PROTAGONIST:What did you find on the gold? NEIL:Three hundred thousand at today’s price, no franks, mould marks. Nothing. Like it came from outer space. PROTAGONIST: Or the future...
Tenet (2020) ScriptSlug