I am unable to duplicate your problem, unfortunately.

The triplets copy for me without any problems.

My first guess would be that the last 16th note is stil in the previous measure, even if it looks like it is in the next measure.

Please specify the steps you've already taken, so that we can rule out things.

Have you tried an Update Layout?

Applied Music Spacing?

Does the problem happen in only one particular document?

Try e. g. in a Document Without Libraries.

Sometimes (with some rhythms and time signatures) if you try to copy by dragging, you get strange results, but if you copy (CTRL-C) a measure and paste (CTRL-V) the measure into another measure, the rhythms should transfer without problems.

My problem occurs when I try to create a score with the song "Red Baron". This particular problem appears like this: I have one bar (4/4) where i have typed four 8th-triplets: triplet no1 consists of two 8ths + two 16ths, triplet no 2 consists of three 8ths, no 3 consists of two 16ths + two 8ths, the last triplet consists of one 8th + two 16ths + one 8th. When I typed it the first time, there was no problem. But when i copy this bar (CTRL+C) and paste it (CTRL+V) into another bar, this bar doesn't accept the last 8th in the last triplet, splits it into two 16ths, and throws the last 16th into the next bar. I have tried copy/paste several times, but it doesn't work. I have also tried music spacing without any success.

If I understand you correctly, all the four triplets should be defined as

“3 Eighths in the space of 2 Eighths”

- right?

My guess is that the third triplet is incorrectly defined as

“3 Sixteenths in the space of 2 Sixteenths”

But without seeing an actual Finale document we can only guess.