I’m currently on 12.9.0.35
when editing a source I double_click on a variable name, then hit Ctrl-R to replace all occurrences.
The editor then tells me it cannot find the variable name 
No replacements occur. The big issue is: this happens intermittently. It does not seem to reproduce consisitently.
Would you plese double-check to see if it reprodcuces on your side?