Odd, I can't reproduce it now either. If it happens again I'll try to get a recording of it.
I can confirm the same behavior on 1.8.7 on 64-bit Linux, Java 8.
Floating tripwire with a frame occupying one of the string blocks does not deactivate when it should.
Tripwire on a surface with a frame occupying one of the string blocks does deactivate.
I can confirm this behavior.
Still happens in 1.9