4iiii HRM dropping/reconnect continually

Hi all - first ride with the 1040 Solar running 12.14 and all was great, other than my 4iiii HRM continually dropping out and reconnecting. Has anyone else experienced this, or are do you have any thoughts on how to try and fix? I removed and added back the sensor - same issue. The HRM worked beautifully with my 1030 (including 332km ride on Monday!) and the 1000 before that, so it feels squarely like a 1040 issue.