1. Sinaiticus and Vaticanus exclude it despite being from 4th century
2. Eusebius, Jerome, Severus and Hesychius suggest Mark never wrote it
3. 23 New Testament manuscripts have anomalies on these verses such as extra endings or doubts the verses are authentic

I think this is an interesting one. Perhaps God allowed it in for a reason.

I definitely wouldn't compare Mark's ending to 1 John 5:7 or Acts 8:37 which were quite clearly added later. Thanks for sharing your thoughts.