Strange tempo change, one note only

[My first post here, so let me know if I should do anything differently.]

In the attached score, the tempo changes spontaneously from 130 bpm to 65 bpm for one note only, the first note in Measure 50, then returns to 130. I have tried setting the tempo for that measure to 130, and I have tried deleting and re-entering the entire measure from scratch (not copying and pasting), but even in the newly entered measure the same problem occurs. As indicated in the note on the score, the intended notes are half note + 8th note, but because of the tempo problem I have revised it to quarter note + 8th note so the playback sounds as intended. I have created several other scores with mixed meters and various tempo changes without this problem occurring. I’m a complete newcomer to MS 4.n; all of my scores were created and edited until now using MS 3.6.2. I just installed v. 4.6.3.252940956, rev. 5d3c963 on my brand-new laptop. The tempo problem does not occur playing the same file on v. 3.6.2. Any ideas?