evalv3 regression involving embeddings and unifying extra fields

[mvdan]

# -- evalv2 --
env CUE_EXPERIMENT=evalv3=0
exec cue export out.cue
stdout 'out.*true'

# -- evalv3 openinline=1 --
env CUE_EXPERIMENT=evalv3=1
env CUE_DEBUG=openinline=1
exec cue export out.cue
stdout 'out.*true'

# -- evalv3 openinline=0 --
env CUE_EXPERIMENT=evalv3=1
env CUE_DEBUG=openinline=0
exec cue export out.cue
stdout 'out.*true'

-- out.cue --
#Foo: {
	_data: Foo: true
	_data: _ | close({Extra!: optional?: _})

	_dataWithExtra: _data & {Extra: {}}

	out: _dataWithExtra.Extra != _|_
}

{#Foo}

As of e311d45, evalv2 prints out: true, but evalv3 with or without openinline prints out: false:

# -- evalv2 -- (0.011s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue export out.cue
[stdout]
{
    "out": true
}
> stdout 'out.*true'
# -- evalv3 openinline=1 -- (0.023s)
> env CUE_EXPERIMENT=evalv3=1
> env CUE_DEBUG=openinline=1
> exec cue export out.cue
[stdout]
{
    "out": false
}
> stdout 'out.*true'
FAIL: repro-evalv3.txtar:10: no match for `out.*true` found in stdout
# -- evalv3 openinline=0 -- (0.022s)
> env CUE_EXPERIMENT=evalv3=1
> env CUE_DEBUG=openinline=0
> exec cue export out.cue
[stdout]
{
    "out": false
}
> stdout 'out.*true'
FAIL: repro-evalv3.txtar:16: no match for `out.*true` found in stdout

Reduced with @myitcv from an original reproducer that was related to openinline. We found an underlying regression unrelated to the use of openinline.