Are you sure your circles are stable outside?
There are two cases with stability circles
1 - stable inside
2 - stable outside
Rstab - stability circle radius
Dstab - distance between center of Smith chart and stability circle center
Four cases:
If center of Smith chart is unstable, and Rstab>Dstab, then circle stable outside
If center of Smith chart is stable, and Rstab>Dstab, then circle is stable inside
If center of Smith chart is unstable, and Rstab<Dstab, then circle stable inside
If center of Smith chart is stable, and Rstab<Dstab, then circle is stable outside
You may provide S11 S21 S12 S22 in complex or polar form and let me check
p.s. recommended:
https://www.n5dux.com/ham/files/pdf/Avoiding RF Oscillation.pdf