Ex-Felons may vote in Missouri

In Missouri, if an individual is out of prison and off probation or parole, they may register to vote again:

115.133. 1. Except as provided in subsection 2 of this section, any citizen of the United States who is a resident of the state of Missouri and seventeen years and six months of age or older shall be entitled to register and to vote in any election which is held on or after his eighteenth birthday.
2. No person who is adjudged incapacitated shall be entitled to register or vote. No person shall be entitled to vote:
(1) While confined under a sentence of imprisonment;
(2) While on probation or parole after conviction of a felony, until finally discharged from such probation or parole; or
(3) After conviction of a felony or misdemeanor connected with the right of suffrage.
3. Except as provided in federal law or federal elections and in section 115.277, no person shall be entitled to vote if the person has not registered to vote in the jurisdiction of his or her residence prior to the deadline to register to vote.
(L. 1977 H.B. 101 § 7.010, A.L. 1979 S.B. 275, A.L. 1982 H.B. 1600, A.L. 1983 S.B. 44 & 45, A.L. 1993 H.B. 23, A.L. 2002 S.B. 675, A.L. 2003 H.B. 511)

I don't have a huge problem with this.

once you have served your time, you are entitled to be treated like any other citizen.

2. No person who is adjudged incapacitated shall be entitled to register or vote.

There aren't very specific parameters on this.
People have guardians usually to help watch their money. Sometimes it is because of a head injury,
other times, they have a bipolar disorder and have had unfortunate episodes of spending.
If it happens that someone has gone to the trouble to make sure they get help managing their money, or something, it's possible that they they won't be able to vote.