15.50Improvising over borrowed chords

If progression is:

Cmaj7 → Fm6 → Cmaj7

do not blindly stay in C major over Fm.

Fm contains:

A♭

So your melody should acknowledge that altered note.

Start with chord tones:

Fm6:

F A♭ C D

Then resolve:

A♭ → G

when C returns.

That's enough to make the improvisation follow the harmony.