George Kadianakis:
I took a brief look at the relevant code, and I'm also not sure what's causing the issue. More digging required.
Toralf Förster:
Something I should worry about ?
I think one should emphasize that relay operators shouldn't worry about it since it's client(s) [hopefully not little-t-tor one(s)] who send such cells. Thanks everybody reporting about this issue!
-- Ivan Markin