Using a fish on an ocelot causes it to enter breeding mode. (Using additional fish causes the number of fish in the player's hand to appear to decrease, but clicking on the relevant inventory slot fish causes the lost fish to reappear.) When two ocelots enter breeding mode, they continue to flee from the player, but will breed if the player hides out of sight.
After breeding, the parents and child will continue to flee from the player, and may despawn. The "Trusting" NBT is never set from 0b to 1b. Manually setting the tag using commands causes the ocelots to stop fleeing and become persistent, as expected.
Related issues
is duplicated by
Comments

Can confirm in 20w51a.

Cannot reproduce this in 21w03a. When feeding a sneaking ocelot fish makes it trusting.
Is this not working for you?
However, what I can reproduce is:
You can feed non-sneaking (fleeing) ocelots fish, causing the ocelot to enter breeding mode
❌ This is confusing because the player cannot tell apart breeding hearts from trusting hearts; it should only be possible for trusting ocelotsYou can feed non-sneaking (fleeing) ocelots in breeding mode fish
❌ This causes a client-side desync because server-side the fish is not actually consumed
Does this describe the issue you are experiencing? If so, may I change the report accordingly?
In 1.16.2 RC1