TLDR; Tooltip requires forwardRef for custom react components to work. Icons don't use forwaredRef.
Use-case example (expected):
import CopyIcon from '@gravity-ui/icons/Copy';
import { Tooltip } from '@gravity-ui/uikit';
<Tooltip content={'Copy'}>
<CopyIcon />
</Tooltip>
But it's not possible now, since icons don't use forwardRef.
The workaround for now is (Icon from uikit has forwadRef):
import CopyIcon from '@gravity-ui/icons/svgs/copy.svg';
import { Tooltip, Icon } from '@gravity-ui/uikit';
<Tooltip content={'Copy'}>
<Icon data={CopyIcon} />
</Tooltip>
But it requires something like svgr for svg import to work and is more cumbersome.