Seems like an interesting feature here.
Reading that doc link you sent, try writing your multiple product filter like this: filter.v.option.color=red&filter.v.option.size=L
The snippet you wrote here: ?filter.p.tag=FRAME+SHAPE_ROUND,SIZE_SMALL , with the comma, I believe only applies to categories in the same realm (color, size). But since “shape” and “size” are two different categories, that may be why it did not work.
Let me know if the snippet I gave you was helpful. Thanks.