Triplet in 3/8 time has four notes?

This is musically correct (3 in the time of 2 plus 1).

Is the problem that they are all beamed together?

If see Beams: https://musescore.org/en/handbook/beam for how to change the beams.

I know that if I were reading that music, I would be at-least puzzled by it.   I would want to see the quaver visually-separate from the triplet, not beamed to it.   (I don’t read enough published music in odd time-signatures to know if this is conventional.)

Confirmed for 2.0.1 on Win7. While numerically and technically 'correct', this is improper notation and the program should not do it this way by default (if at all).

As shown above, It's easy enough to correct incidences of this by using the Beam Properties palette from the F9 menu, but it really should be addressed in the code, IMO. Would you like to file an Issue using the Issue Tracker?