Proud and prideful

[TonyChanYT]

Proverbs 16:

18 Pride goes before destruction, and a haughty spirit before a fall.

Pride in the sense of haughtiness is a sin, 1 John 2:

16 the lust of the flesh, the lust of the eyes, and the sinful pride of life

Proverbs 8:

13 I hate pride and arrogance, evil behavior and perverse speech.

In English, there is a difference between "proud" and "prideful". "Prideful" is negative; "proud" could be positive. A parent can be proud of his kids without being arrogant. A person may be proud of his achievement after a long struggle.
The Father praised the Son in Matthew 3:

17 a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”

A master praised his servant in Matthew 25:

23 His master replied, ‘Well done, good and faithful servant! You have been faithful with a few things; I will put you in charge of many things. Come and share your master’s happiness!’

Corinthians boasting about themselves was bad, 1 Corinthians 5:6

Your boasting is not good. Do you not know that a little leaven works through the whole batch of dough?

But then, Paul boasted about the Corinthian congregation in 2 Corinthians 7:

4 Great is my confidence in you; great is my boasting on your behalf. I am filled with comfort; I am overflowing with joy in all our affliction.

Paul likened confidence with good boasting.
Is being proud sinful?
No, when you are thankful and acknowledge God for it. Yes, otherwise.