Thanks for this example. I can see what’s going on here: basically the presence of the rests in the set of notes you’re trying to re-tupletify is throwing things off.
You have a couple of options while the program behaves this way, I think. The easiest one is to temporarily add dummy notes to fill in the gap in the span that you want to become encompassed by the tuplet, then remove them after you have made the tuplet successfully.
The slightly less easy one is to switch off Insert mode, select the span that you want to be added to the tuplet, and create the tuplet: this will not delete any music, but will not “pull” the final G 16th note back into the tuplet. Select it and hit Alt+left to move it back into the tuplet. You will be left with an 8th rest between this 6:4 tuplet and the next one: show the caret after the rest and hit Backspace to pull the remaining music back into the right spot.
Obviously this should be easier to do. Simplest of all would be the ability to directly change the ratio of existing tuplets, and perhaps even to come up with a way of directly combining or splitting them. These are definitely things we want to work on further in future versions.