I have noticed the same thing. In general I have just gone back and
selected the correct value. It seems to clear up the issue the second
time. I wasn't thinking bug, more of not clearly selecting a single value
from list. You may be correct.
Sent from my mobile device... Please excuse typos.
After careful inspection, I think I finally figured out why this happens. If I accidentally clicked into this field twice (fast), the first value is place and the cursor is set immediately after that text so the next values is put in right after that. So there is no 'bug', just some fast clicking issue I need to be more careful about. I experimented and enabled the field validation to only be from the value list; it worked OK but then you get the funny FM error dialog that asks to revert the field so I just disabled that option again.
It's definitely not a bug. For fields populated from a dropdown list, I find it helpful to check "select entire contents on entry" in the Data Inspector. That avoids this problem almost entirely.