1. Different emotional investment: Your ex may have invested less emotion and attention in you, so they are unwilling to give too much material effort, manifested as stinginess. For the current position, due to the freshness and investment of emotions, he is more willing to show his sincerity and love through generosity.
2. Differences in Relationship Stages: In your relationship, you may have entered a relatively stable or flat stage, and your ex may believe that there is no need to maintain the relationship through material means anymore. Being generous is a strategy to attract and solidify a new relationship with someone who is in the early stages of their relationship or in the honeymoon period.
3. Personal growth and change: People's values and behavior patterns will change over time and personal experiences. The stinginess of the ex in front of you may just be a reflection of his attitude towards life or financial situation at that time. And his generosity towards the current position may be due to his own growth and changes in economics, emotions, or worldview, learning to cherish and express love more.