Simulation of TDR using LTSpice
We can sort out the times here.

At the probe point, the incident signal will be at 0.5(on) (as it is) until the signal has traversed TLA (it is a 1:1 voltage divider during transit), then had a discontinuity at TLB and returned from that point.

Note that you must use the effective impedance depending on where the signal is; at &lt; 10nsec (for your probe point), TLB does not exist.

As the traversal time (round trip) in TLA is 10ns, that is the time you will see the discontinuity reflected from TLB at the probe point. When the signal is returning from an impedance higher than the source, it returns in-phase. The reflection co-efficient at the TLB entrance is (75-50)/(75+50) = 0.2, and we do indeed see the reflection increase by this amount.

At this time, the signal has already moved through TLB by 5 ns, and therefore it will take another 5nsec (outbound through TLB) + 10 ns (return through TLB) + 5 nsec (return through TLA) = 20 nsec for the next discontinuity to show up at the probe point for a total time of 30 nsec.

Just why you see another discontinuity into something larger than the specified load impedance (reflection co-efficient of -0.8, approxiamtely 5 ohms) is interesting and may have something to do with the specifics of each model.