Thanks for bringing this up. I actually never paid attention to this detail throughout all these years! Just did a scan of some of my square and rectangular minis for you, it's interesting what I found:
RECT:
- My so black chevron lamb rect mini (21 series) is fabric lined inside the zipper pocket
- My recent gray lamb rect mini (28 series) is leather lined inside the zipper pocket
SQUARE:
- My taupe square lamb mini (20 series) is fabric lined inside the zipper pocket
- My black square caviar mini (25 series last caviar batch) is leather lined inside the zipper pocket
So then I pulled out my other minis in between this time frame:
- My black rect caviar mini (23 series burgundy interior) is leather lined inside the zipper pocket
- All my other 21 series lamb or caviar rect minis are fabric lined inside the zipper pocket
So I can conclude that Chanel must have updated the material sometime, either starting at 22 series or 23 series. Perhaps others can help narrow down to the exact season. I do remember that prices started to increase significantly for minis around that time, perhaps changing to use leather was to justify the increase at the time. (Of course we now know that prices just kept increasing and there is no further justification!)