Uneven triplet spacing when adding second part

I'm reposting this with a shorter sample since the last post about 24 hours ago didn't go through moderation apparently. I hope this one will.

When I create triplets in one part, the triplets are evenly spaced.

However, when I start adding another part, which does not use triplet rhythm, the triplets in the existing part change, and are no longer displayed evenly.

I have attached an example. You can see the result in the top "system" in the treble part, with the uneven triplets.

In the second "system", where I haven't entered the bass part yet, the triplets in the treble are properly even.

What I expect is for a triplet of three 8th notes in the treble part to align and be spaced evenly against two "regular" (non-triplet) 8th notes in the bass part.

The current behavior in Musescore doesn't just make the score hard to read, it also causes the print to be wider, and my score, when complete, ends up not fitting on two pages anymore, due to the misalignment of notes.

Is there any way to fix this, and if so, how ?

You can see how I expect the score to look at http://ks4.imslp.net/files/imglnks/usimg/d/df/IMSLP132660-WIMA.e0b8-Ram…, page 3. This is consistent with the original manuscript by Rameau.