ecryptfs causes Cargo to rebuild when it shouldn't

What I did:
- `cargo clean`
- `cargo build` => everything is rebuilt, that's OK
- `cargo build` again => some crates are rebuilt, that's **not OK**
- `cargo build` again => nothing is rebuilt, that's OK

Happens on my laptop running Debian/testing, rust 1.40 from rustup, 8Gb RAM.  
Doesn't happen on my desktop, sames OS, same rust, 20Gb RAM.  

How can I help and provide debug info ?