Unexpected output when using FindInLine in a json object

In a web script I am doing this per track

...snip...
OutputTo "MOVEMENTNAME"
json_select "name"
FindInLine ":"
SayRest
Say "|"
...snip...

But on the next pass, the last letter from the previous is added in front of it.

Eg. First time it is correct: "Allegro", but the next one becomes "oAdagio", then "oFinale".

I found a workaround, by adding a space after the ":" it works properly.

I'd need a minimal working example if you think you've found something worth investigating. It's expected that FindInLine also works with single-character find strings.