I finally figured out how this is happening. You are apparently using a setting that I never use. Namely the gateway test "Number of Graded Submissions per Test" option being set to something other than 1. I am guessing you left this at the default value of 0 when you tried this. In this case, the javascript auto submissions start 10 seconds before the end of the grace period, and occur repeatedly until the full grade period is over.
See https://github.com/openwebwork/webwork2/pull/1953 for the fix.