I don’t know. I have seen though, that Gods word was very sweet to me at first but it became bitter turmoil in me for many, many years. Then it turned sweet again and what was bitter to me when I was so miserable in my uncontrolled flesh is now wonderful promises and great hope.
I also see a connection to the Passover lamb having to be eaten with bitter herbs.
But as for what is sealed up, no clue?
There are a lot of things you can think of as some mix of sweet and bitter: self-reflection, the truth, etc. But God doesn't discourage these things, God encourages them. If God seals something away, it's something else.
For a similar example of a food analogy:
Jeremiah 31:29-30
“In those days they will not say again,
‘The fathers have eaten sour grapes,
And the children’s teeth are set on edge.’
But everyone will die for his own iniquity; each man who eats the sour grapes, his teeth will be set on edge.
A sour grape will cause you to die for your iniquity. So a bitter stomach is an idea so destructive to your conscience it will kill your soul if you are not ready to receive it.