I experienced the following bug with PM 6 build 4538(51e5663):
I was trying to type an e with an umlaut (ë) into the headline of a single photo's metadata using the IPTC info (not the template/stationary pad). I get into the metadata by selecting the photo and hitting the `I' key on my keyboard. Then I went into the headline section of the metadata and typed a name with the character in it, using "alt+0235" on the keyboard number pad to type that special character. This apparently triggers a few actions within photo mechanic, the end result being that the original photo had a 2 star rating applied to it, the next photo was selected, and then the `ë' character was placed at the start of that next photo's headline in the metadata.
I cannot type that special character anywhere in an individual photo's metadata without this happening. I usually edit captions in batches using the metadata template, and the special character is ok there. The only solution I found was to copy and paste that specific character into the correct photo's metadata.