Hmm interesting! Are you sure the continuity between pin 2 SWIM and pin4 UH7 is good (i.e. as close to 0 Ohms as possible)?

I suppose the scope adds a bit of capacitance to the circuit, so might act as a filter or maybe create a bit of a delay. Is the signal on the scope clean or is there lots of noise? Might also be a ground issue on the IC, did you check for good / noise ground and good power?

I guess you could always put a cap of same capacitance as the scope leads in the circuit to see if that fixes it, but it shouldn't need it so there must be a problem somewhere...

Keep up the good work