That really depends on whether you specify a timestamp when you set a value to the output tag. If not, it should default to ExeTime. if you are setting the output value with something like:
OutputTag1.Value = 100
Then all the output tags should have the same output time. You can also verify by checking the ResultTimestamp property of the output tags. This property only works for tags that are configured as output tags.
And yes, the compression setting of the output tags can be a reason for this as well.
Yes, for scheduled tags the timestamp should be same for all output tags because it writes to all of them at end of execution. Only if compression is the we might loose some values. One should not have any exception and compressions settings for any ACE output tags. Please let us know how it goes.