Hmmm, their wording is not very clear, but they specifically say "If your PV modules are wired in series, and your design includes a hard angle (some facing east, and some facing west), the early morning and late afternoons may see a short period where there is a complete drop-off of production for the heavily shaded PV-Modules. This is a normal function of Optimization"
I take that to mean complete drop off of the string that contains the "heavily shaded PV-Modules"; I could be wrong, but if they didn't mean that, all they are saying is that heavily shaded panels won't produce power (which is kind of obvious).
But looking at the article I linked to, in example 2, where there are is one string of 12 panels, 6 south and 6 west, they specifically say...
"Tigo recommends dividing the string into two separate strings in series going into separate MPPTs"