I checked the "please enter 4 more characters..." text and found out this to be a bug. Actually the string comes from the vibe course module plugin but in the front end plugin the string is not called in one place and therefore causing this issue only with the assignments.
I am marking this as a bug and it will be fixed in the next update of wplms front end plugin.
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Please do let me know more bugs we'll fix it and release an update for it. Its good that you share the bugs with us because then we got to fix it. It doesn't bother us because its also beneficial for us.