The data shows you have 36 distinct groups. There are three ways to address this.
1. Increase MaxLegendArea. You already increased this to 45. You can go higher.
2. You can reduce the displayed LINELENGTH in the legend. LineLength=24 may work. For solid lines, lower value will work.
3. Another way is to separate your year and month into new columns. Now, you can color your lines by month (12) and use linepattern to show the year (3) using GROUPLC and GROUPLP options (see some examples in Graphically Speaking - Spaghetti Plot). Now, you need two legends, one for the color, which can be a short line (linelength=10), or just TYPE=COLOR, and one for patterns (3). Note, you still need the fin_priority group column.
series x=seniority y=vintage3 / group=fin_period grouplc=month grouplp=year lineattrs=(thickness=2);
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