Ampersand (&) gets doubled in GREP Style?

I'm using InDesign CC 2019 (14.01), and I have a GREP style applied to a paragraph style, the pattern contains an ampersand (e.g. 'This & that').

I've been having intermittent problems where the pattern suddenly changes unexpectedly to 'This && that'. I even tried escaping the ampersand ('This \& that'), but it still changed the style. It doesn't seem to do it all the time, but I can't determine under what circumstances it made the change. I tried making a new document and entering a new style, and it changed it after I created the style, but I was unable to repeat it on subsequent tries.

Is this a known bug? Is there a way for me to be sure it won't change when I send the job to the printer?