If God withholds a specific provision from someone, and that person knows in their heart that this is good from God, but they feel displeasure due to people's gloating and words, is this considered discontent and a lack of submission to God's decree, and is repentance obligatory upon them?

Whoever is patient with Allah's decree, submitting to His judgment, neither despairing nor complaining, then there is no sin upon him. Although the more complete state is to be pleased with all that Allah destines and decrees, this pleasure is not obligatory; rather, it is only recommended. The correct view is that what is obligatory is patience.