Shaykh ʿUbayd al-Jābirī was asked:

Is there a difference between it being said: So and so is a ‘person of innovation (صاحب بدعة)’ and: ‘Innovator (مبتدع)’?

He answered:

According to what I have understood from the people of knowledge there is a difference between them. A ‘person of innovation’ is like the one who fall into an innovation. And an ‘innovator’ is the one upon whom the proof has been established and knows that this is an innovation, that his action is an innovation. Then he became stubborn and arrogant [with respect to it].

Source: Miraath.Net
Translated by Abū ʿIyāḍ Amjad Rafiq @abuiyaadsp

السؤال : شيخنا – أثابكم الله – هل هناكَ فرقٌ بينَ أَن يُقال فلانٌ صاحبُ بِدعة وفلان مبتدع؟ الجواب : حَسبَ ما فهمته من أهلِ العلم أن بينهما فرقًا، فصاحبُ بدعة مثل واقع في بدعة, و مبْتَدِع هو الذي قامت عليه الحُجَّة وعَلِمَ أَنَّ هٰذا بِدعة؛ أَنَّ عَمَلَهُ بدعة, فعانَدَ وكابَر. نعم.